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Self-luminous devices and melatonin suppression in adolescents

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Abstract

Self-luminous devices, such as computers, tablets and cell phones can emit short-wavelength (blue) light, which maximally suppresses melatonin. Melatonin is a hormone that starts rising approximately 2 hours prior to natural bedtimes and signals darkness and sleep to the body. The present study extends from previously published studies showing that light from self-luminous devices suppresses melatonin and delays sleep. This is the first study conducted in the home environment that investigated the effects of self-luminous devices on melatonin levels in adolescents (age 15–17 years). Results show that 1-hour and 2-hour exposure to light from self-luminous devices significantly suppressed melatonin by approximately 23% and 38% respectively. Compared to our previous studies, these results suggest that adolescents may be more sensitive to light than other populations.
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Self-luminous devices and melatonin
suppression in adolescents
M Figueiro PhD and D Overington
Lighting Research Center, Rensselaer Polytechnic Institute, Troy,
New York, USA
Received 11 March 2015; Revised 7 April 2015; Accepted 9 April 2015
Self-luminous devices, such as computers, tablets and cell phones can emit short-
wavelength (blue) light, which maximally suppresses melatonin. Melatonin is a
hormone that starts rising approximately 2 hours prior to natural bedtimes and
signals darkness and sleep to the body. The present study extends from previously
published studies showing that light from self-luminous devices suppresses
melatonin and delays sleep. This is the first study conducted in the home
environment that investigated the effects of self-luminous devices on melatonin
levels in adolescents (age 15–17 years). Results show that 1-hour and 2-hour
exposure to light from self-luminous devices significantly suppressed melatonin
by approximately 23% and 38% respectively. Compared to our previous studies,
these results suggest that adolescents may be more sensitive to light than other
populations.
1. Background
Retinal light exposures during the night
suppress melatonin production in a
dose-dependent manner.
1
The degree of sup-
pression is positively correlated with the
irradiance and duration of light exposure.
1
Under strict laboratory conditions and in a
dose-response dependent manner, exposure to
100 lux of a cool white fluorescent light will
cause 50% of the melatonin suppressive effect
induced by exposure to 10,000 lux,
1
demon-
strating the high sensitivity of the circadian
pacemaker to light. The circadian system, as
measured by acute melatonin suppression and
phase shifting of dim light melatonin
onset (DLMO), is maximally sensitive to
short-wavelengths (peak k460 nm). Light
information is perceived by the retinal photo-
receptors that project to the suprachiasmatic
nuclei via the retinohypothalamic tract, and
then via the superior cervical ganglion to the
pineal gland, to control melatonin produc-
tion.
2–6
It has been shown that some degree
of melatonin suppression was observed
after 90-minute exposure to 2 lux of
470-nm light, which emits most of its energy
close to the peak sensitivity of the circadian
system.
7
In addition to the direct effects of light on
suppressing melatonin production, light also
affects the timing of melatonin production via
the circadian pacemaker.
8
For example, light
stimuli applied within a few hours before the
midpoint of the melatonin secretion episode
will delay the phase of the circadian pace-
maker, while light stimuli applied within a
few hours after this midpoint will advance the
phase. Thus, inappropriate light exposure can
change the timing of melatonin production to
an abnormal phase.
Morning light (i.e. light after the minimum
core body temperature) promotes entrain-
ment to the solar day and, therefore, supports
Address for correspondence: Mariana G. Figueiro PhD,
Lighting Research Center, Rensselaer Polytechnic Institute,
21 Union Street, Troy, NY 12180, USA.
E-mail: figuem@rpi.edu
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daytime alertness and performance, while
light in the evening and in the early part of
the night may lead to acute suppression and
delay the timing of melatonin production,
both of which are hypothesised to be detri-
mental to sleep quantity and quality. Sleep
deprivation has been associated with diabetes
and obesity.
9
Melatonin suppression by
exposure to light at night and circadian
disruption resulting from irregular light/dark
patterns have been associated with more
serious health risks, such as cancer.
10
In
adolescents, chronic sleep deprivation result-
ing from late bedtimes and early wake times
has been associated with reduced school
performance, increased risk of diabetes and
obesity, mood disorders and drug
addiction.
11–13
Self-luminous devices, such as computers
and tablets that use light-emitting diodes
(LEDs), emit short-wavelength (blue) light,
close to the peak sensitivity of the circadian
system. Advances in technology have led to
larger and brighter screens, and these devices
are being used for longer periods of time in
the evening, when melatonin, a hormone
signaling darkness to the body, starts rising.
According to the U.S. Census Bureau, more
than 75% of households have a computer
14
and presumably use these devices in the
evening hours.
Cajochen et al.
15
examined the effects of
LED-backlit computer screens on circadian
rhythms and cognitive performance in a study
of 13 participants. Two 5-hour experimental
sessions were held with one of two computer
screen types: (1) white LED backlit computer
screen; (2) non-LED backlit screen (cathode
fluorescent lamp). The screens had similar
illuminance/visual comfort ratings, but dif-
ferent spectral compositions. Results showed
that a 5-hour exposure to a LED backlit
computer screen significantly suppressed
melatonin, reduced sleepiness levels and
enhanced cognitive performance (e.g. atten-
tion, memory) compared to a non-LED
backlit screen. Although melatonin levels
still rose over the course of the night, they
did not rise as steeply as when participants
experienced the non-LED backlit computer
screen condition. More recently, Chang
et al.
16
showed that compared to reading a
paper book in near darkness, electronic read-
ers acutely suppressed melatonin, delayed the
onset of evening melatonin and negatively
affected sleep quality.
Figueiro et al.
17–19
have performed three
studies investigating the impact of three types
of self-luminous devices (computer, iPad and
television) on evening melatonin levels. In the
first study, they investigated the effect of a 2-
hour exposure to cathode ray tube (CRT)
computer screens on acute melatonin sup-
pression in college students.
17
Twenty-one
participants experienced three test conditions:
(1) computer monitor only (experimental
condition), (2) computer monitor viewed
through goggles providing 40 lux of short-
wavelength (blue; peak k470 nm) light at
the cornea from LEDs (‘true-positive’ experi-
mental condition), and (3) computer monitor
viewed through orange-tinted safety glasses
that filtered optical radiation below 525 nm
(‘dark’ control condition). Saliva samples
were collected from participants at 23:00,
before starting computer tasks, and again at
midnight and 01:00 while performing com-
puter tasks under all three experimental
conditions. Melatonin concentrations after
2-hour exposure to the blue-light goggles
were reduced compared to the dark control
and to the computer monitor only conditions.
Although not statistically significant due to
high variance in suppression among partici-
pants, the mean melatonin concentration
after exposure to the computer monitor was
reduced by about 16% relative to the dark
control condition.
17
Using the same experi-
mental design, they showed that iPad use did
not significantly suppress melatonin after
1 hour (mean suppression ¼7%), but after
2-hour exposure, suppression was statistically
2Figueiro and Overington
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significant (mean suppression ¼23%).
18
In
their third study, Figueiro et al.showedthat,
compared to the dark control, melatonin levels
continued to rise after 90-minute exposure to
70-inch televisions, suggesting that light
coming from televisions is not strong enough
to suppress melatonin in the evening.
19
This field study extends those previously
published and is the first study conducted in
the home environment that investigated the
effects of self-luminous devices on melatonin
levels in adolescents (ages 15–17 years). Two
test conditions were employed: (1) self-
luminous devices viewed through orange-
tinted glasses (optical radiation5525 nm 0)
and (2) self-luminous devices viewed without
orange-tinted glasses. The orange-tinted
glasses served as a ‘dark’ control condition
since they removed short-wavelength radi-
ation that maximally suppresses melatonin
production while participants performed
tasks using self-luminous devices. Since, how-
ever, self-luminous devices deliver different
corneal irradiances depending on the type,
size and brightness of the display and on the
postures of the viewers, and since each
participant had a different type of self-
luminous device, each participant was asked
to wear a pendant Daysimeter,
20
a calibrated
light meter that monitored circadian light
exposures over the course of the data collec-
tion period.
2. Method
2.1 Participants
Twenty adolescents were recruited into this
within-subjects study. The age range of the
participants (7 males and 13 females) was
between 15 and 17 years. Eligibility for the
study required participants to be free of any
major health problems, such as cardiovascu-
lar disease, diabetes or high blood pressure.
Potential participants were excluded from the
experiment if they were taking over-the-
counter melatonin or prescription medication
such as antidepressants and sleep medicine.
They were not excluded from the experiment
if they were taking oral contraceptives.
Participants were asked to self-report any
eye diseases or colour blindness. Potential
participants who stated they had an eye
disease were excluded from the study.
2.2 Experimental and control conditions
Two lighting conditions were employed on
two consecutive days in November 2013 in
Albany, NY, USA. In both experimental
conditions, participants were asked to use
self-luminous devices starting 3 hours prior to
their normal bedtimes. During the first night,
participants were asked to use any kind of
self-luminous devices while wearing orange-
tinted glasses (SAF-T-CUREÕOrange UV
Filter Glasses) that filtered out all optical
radiation below approximately 525 nm that
could otherwise be effective for suppressing
nocturnal melatonin. This served as the ‘dark’
control condition. On the following night,
participants again performed tasks displayed
on self-luminous devices for 3 hours prior to
bedtimes, but they viewed the self-luminous
devices through the orange-tinted glasses for
the first hour only, after which they were
asked to remove the orange-tinted glasses and
continue to use the self-luminous devices
for another 2 hours. The participants were
allowed to use computers, tablets, e-readers,
televisions and cell phones. They were asked
to use the same type of self-luminous devices
both nights, even though on one single night,
they were allowed to use more than one type
of self-luminous device. The ambient lighting
in the room in which they were performing
the tasks was the same during both nights.
2.3 Daysimeter measurements
To estimate the light exposures that par-
ticipants were actually experiencing while
using the self-luminous devices without the
orange-tinted glasses, they were asked to wear
a pendant Daysimeter
20
during the study.
Melatonin suppression in adolescents 3
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Based on Figueiro et al.,
20
measurements
obtained with pendant Daysimeters are
more closely related to measurements
obtained with the Daysimeter located near
the eye than when the device is placed on the
wrist. Moreover, compliance rates are better
when wearing the pendant Daysimeters than
when wearing a head-mounted device. Briefly,
light sensing by the Daysimeter is performed
with an integrated circuit (IC) sensor array
(Hamamatsu model S11059-78HT) that
includes optical filters for four measurement
channels: red (R), green (G), blue (B) and
infrared (IR). The R, G, B and IR photo-
elements have peak spectral responses at
615 nm, 530 nm, 460 nm and 855 nm, respect-
ively. The Daysimeter is calibrated in terms of
orthodox photopic illuminance (lux) and of
circadian illuminance (CL
A
). CL
A
calibration
is based upon the spectral sensitivity of the
human circadian system proposed by Rea and
colleagues.
21
From the recorded CL
A
values it
is then possible to determine the circadian
stimulus (CS) magnitude, which represents
the input-output operating characteristics of
the human circadian system from threshold to
saturation. Values of CS are numerically
equal to the amount of expected melatonin
suppression from the CL
A
exposures accord-
ing to the model by Rea et al.
21
2.4 Protocol
Every subject participated in the experi-
ment on two consecutive nights. All partici-
pants were asked to maintain their regular
sleep schedules for the week prior to the
experimental sessions. Participants were also
asked to refrain from napping and using
products containing caffeine (coffee, tea,
chocolate or soda) starting at 10:00 a.m. on
the days of the experiment.
Prior to starting the study, each participant
was given the Daysimeter and saliva tubes.
On the days of the study, participants were
asked to wear the Daysimeter from the time
they woke up until the end of data collection,
which was just prior to their usual bedtime.
Participants were asked to put on the
orange-tinted glasses and start using their
self-luminous devices (computers, tablets,
e-readers, televisions and/or cell phones)
3 hours prior to their usual bedtimes. At the
end of the first hour, participants were asked
to collect one saliva sample (T1). On the first
day of the study, participants were instructed
to continue wearing the orange-tinted glasses
and to collect two additional saliva samples:
2 hours (T2) after they started wearing the
orange-tinted glasses, and 3 hours (T3) after
they started wearing the orange-tinted glasses,
just prior to bedtime. On the second day of
the study, participants were asked to follow
the same protocol, but they were instructed to
remove the orange-tinted glasses right after
the first saliva sample collection; therefore the
last two saliva samples were collected while
they were being exposed to the light from
their self-luminous devices. Participants were
asked to use the same self-luminous devices
and to keep the same data collection times on
both experimental nights.
Saliva samples were collected using the
Salivette system (ALPCO Diagnostics, Salem,
NH, USA). To provide a saliva sample for
assessment of melatonin concentration, each
participant removed a self-contained plain
cotton cylinder from the plastic test-tube,
moved the cotton around in the mouth until
saturated with saliva, and returned the
saturated cotton cylinder to the plastic test-
tube. Each participant refrigerated the saliva
sample and the experimenter collected the
test-tubes at the end of the second day and
delivered them to the Lighting Research
Center, where the samples were then spun in
a centrifuge for 5 minutes at 1000 gto remove
the saliva from the cotton cylinder. The
cotton cylinder was discarded and the saliva
sample was immediately frozen (20 8C). The
frozen samples were assayed in a single batch
using melatonin radioimmunoassay kits. The
sensitivity of the saliva sample assay was
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reported to be 0.7 pg/mL and the intra- and
inter-assay coefficients of variability (CVs)
were 12.1% and 13.2%, respectively.
2.5 Data analyses
Two participants did not provide enough
saliva at the last data collection period so
statistical analyses include data from 18 par-
ticipants who had complete data sets. After
downloading the Daysimeter data, we per-
formed visual inspection of the data to verify
for compliance or any device malfunction.
Device failure was determined when readings
were extremely high for nighttime hours
(e.g. light levels above 2000 lux) and non-
compliance was determined when activity
readings were zero. Of the 18 participants,
data from 11 participants were included in the
Daysimeter analyses. In order to determine
the amount of light exposure received dur-
ing the data collection period, we are report-
ing the mean and median photopic light levels
(lux) and CS values for the last 2 hours in
which participants were using self-luminous
devices without the orange-tinted glasses.
Although participants were asked to main-
tain a regular schedule, the exact times that
they started and ended data collection on
each day varied. In fact, five participants
started data collection between 30 and
120 minutes later on Night 2 than on
Night 1. Therefore, melatonin concentrations
at T2 and T3 were normalised to the concen-
trations at T1, which was always collected in
circadian darkness, because participants wore
the orange-tinted glasses during the first hour
of the data collection period on both experi-
mental nights.
Based upon data from 18 participants, a
23 factor (Night, one control and one
experimental condition, and Time, three
sample collection times) analysis of variance
(ANOVA) was performed on the un-normal-
ised melatonin concentrations. We also per-
formed a 2 2 factor (Night, one control and
one experimental condition, and Time, two
sample collection times) ANOVA using the
normalised to T1 melatonin concentrations.
In this analysis, the concentrations at T1 were
considered to be 1 and were not included in
the analyses. Where appropriate, post-hoc
two-tailed paired Student’s t-tests were also
performed and a p50.05 criteria was used.
Melatonin suppression was calculated
using the ratio of the melatonin concentra-
tions at T2 and T3 on Night 2 relative to the
melatonin concentrations at T2 and T3
obtained on Night 1 (‘dark’ control condi-
tion). On each night, melatonin concentra-
tions at T2 and T3 were normalised to T1 and
melatonin suppression at each time (i.e. T2
and T3) was calculated using the following
formula:
1M2
M1

where M2 is the normalised melatonin con-
centration at each time on Night 2 and M1is
the normalised melatonin concentration at
each time on Night 1.
A two-tailed, one-sample T-test was used
to determine whether suppression was signifi-
cantly different than zero.
3. Results
3.1 Daysimeter data
The mean standard error of the mean
(SEM) photopic light level from the
Daysimeter data was 87 32 lux and
the median photopic light level was 37 lux.
The mean SEM CS value was 0.04 0.01
and the median CS value was 0.02. This
suggests that predicted suppression after
1 hour exposure, assuming a pupil diameter
of 2.3 mm would be about 4%.
3.2 Melatonin concentrations
For melatonin concentrations, the
ANOVA revealed a significant main effect
Melatonin suppression in adolescents 5
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of Night (F
1,17
¼4.7; p¼0.045; partial
2
¼0.22), a main effect of Time
(F
2,34
¼14.2; p50.001; partial
2
¼0.45) and
a significant interaction between the variables
(F
2,34
¼16.4; p50.001; partial
2
¼0.49). As
shown in Figure 1, melatonin concentrations
were significantly higher on Night 1, when
participants wore the orange-tinted glasses,
than on Night 2, when they used the self-
luminous devices without the orange-tinted
glasses. The mean SEM melatonin concen-
trations were 11.4 1.2 pg/mL on Night 1
and 9.5 0.8 pg/mL on Night 2. In general,
melatonin concentrations increased from T1
to T3. Melatonin concentrations were signifi-
cantly higher at T3 than at T1 (p¼0.001) and
at T2 (p¼0.009). Melatonin concentrations
were also significantly higher at T2 than at
T1 (p¼0.001). The mean SEM melatonin
concentrations were 8.2 0.8 pg/mL at T1,
10.8 1.1 pg/mL at T2 and 12.3 1.2 pg/mL
at T3. Figure 2 shows the mean SEM
melatonin concentrations at T1, T2 and T3
on each experimental night. The mean SEM
melatonin concentrations on Night 1 were
7.5 0.8 pg/mL at T1, 11.7 1.4 pg/mL at T2
and 15.0 1.9 pg/mL at T3. On Night 2, the
mean SEM melatonin concentrations were
9.0 0.9 pg/mL at T1, 9.9 1 pg/mL at T2
and 9.6 0.8 pg/mL at T3. Post-hoc two-
tailed Student’s t-tests revealed that mela-
tonin concentration was significantly higher
(t17 ¼2.4; p¼0.03) on Night 2 than on Night
1 at T1 but significantly lower (t17 ¼3.6;
p¼0.002) on Night 2 than on Night 1 at T3.
On Night 2, the higher melatonin concentra-
tions at T1 (i.e. after participants wore the
orange-tinted glasses for 1 hour) was likely
due to the fact that five participants started
the data collection 30 to 120 minutes later on
the second evening than on the first evening,
and therefore, their melatonin concentrations
at T1 were higher because it was a later time
in the evening.
20
15
10
5
0T1 T2 T3
Night 1 (with orange-tinted glasses)
Night 2 (without orange-tinted glasses)
Melatonin Concentration (pg/mL)
Figure 2 Mean standard error of the mean (SEM) of
the melatonin concentrations at T1, T2 and T3 on Nights
1 and 2. On both nights at T1, saliva samples were
collected 1 hour after participants started using the self-
luminous devices while wearing orange-tinted glasses.
Melatonin concentration was significantly higher
(t17 ¼2.4; p¼0.03) on Night 2 than on Night 1 at T1 but
significantly lower (t17 ¼3.6; p¼0.002) on Night 2 than on
Night 1 at T3. The higher melatonin levels obtained at T1
on Night 1 may be due to the fact that five participants
started data collection on Night 2 between 30 and 120
minutes later than on Night 1. At T2 and T3, saliva
samples were collected 2 hours and 3 hours after
participants started using the self-luminous devices.
Participants wore orange-tinted glasses for the duration
of the data collection period on Night 1. On Night 2,
participants were instructed to remove the orange-tinted
glasses right after the first saliva sample collection
14
12
10
8
6
Melatonin Concentration (pg/mL)
4
2
0Night 1
(with orange-
tinted glasses)
Night 2
(without orange-
tinted
g
lasses)
Figure 1 Mean standard error of the mean (SEM) of the
melatonin concentrations on Night 1, when participants
were using the self-luminous devices while wearing
orange-tinted glasses that filter out optical radiation
below 525 nm and on Night 2, when participants were
asked to use self-luminous devices without the orange-
tinted glasses. There was a statistically significant
(p¼0.045) reduction in melatonin concentrations on
Night 2 compared to Night 1
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For the normalised melatonin concentra-
tions, the ANOVA revealed a significant main
effect of Night (F
1,17
¼19.5; p50.001; partial
2
¼0.53), a significant main effect of Time
(F
1,17
¼16.1; p¼0.001; partial
2
¼0.49) and
a significant interaction between the variables
(F
1,17
¼12.3; p¼0.003; partial
2
¼0.42).
Normalised melatonin concentrations were
significantly higher on Night 1 than on Night
2 and at T2 than at T3. The mean SEM
normalised melatonin concentrations were
1.84 0.2 pg/mL on Night 1 and
1.15 0.07 pg/mL on Night 2 and
1.36 1.2 pg/mL at T2 and 1.63 1.4 pg/mL
at T3.
As shown in Figure 3, normalised mela-
tonin concentrations rose over the course of
Night 1 but did not rise as much on Night 2,
when participants were performing the tasks
on self-luminous devices without the orange-
tinted glasses. Post-hoc Student’s t-tests
revealed that normalised melatonin
concentrations at T3 were significantly
higher than at T2 on Night 1 (t17 ¼4.8;
p50.001) but not on Night 2 (t17 ¼0.12;
p40.05), suggesting that on Night 2, when
participants were using the self-luminous
devices without the orange-tinted glasses,
melatonin levels did not rise as much as on
Night 1. Post-hoc Student’s t-tests also
revealed that normalised melatonin concen-
trations were significantly higher on Night 1
than on Night 2 at T2 and at T3 (t17 ¼3.3;
p¼0.004 and t17 ¼4.8; p50.001,
respectively).
3.3 Melatonin suppression
The two-tailed, one sample t-test revealed
that the normalised melatonin suppression
was significantly different than zero at T2
(t17 ¼3.7; p¼0.002) and at T3 (t17 ¼6.3;
p50.0001). The mean SEM suppression
was 22.8% 6% at T2 and 38.3% 6% at
T3. These results show that exposure to
1 hour and 2 hours of light from self-
luminous devices significantly suppressed
melatonin in adolescents.
4. Discussion
The present field study results extend from
laboratory data,
15–19
and show that using
self-luminous devices for 1 or 2 hours in the
evening can reduce melatonin concentrations
in adolescents. These findings are consistent
with those from the previous studies discussed
above,
15–19
except that the present study was
conducted in the field, did not select a specific
self-luminous device, and all participants were
adolescents. One of the most striking findings
from the present study was that adolescents
(ages 15 to 17 years) seem to have a
heightened sensitivity to evening light for
acute melatonin suppression. The amount of
suppression compared to the predicted sup-
pression from the Daysimeter measurements
observed in the present study was much
2.5
2.0
1.5
1.0
0.5
0
Night 1 (with orange-tinted glasses)
Night 2 (without orange-tinted glasses)
T1
Normalised
Melatonin Concentration (pg/mL)
T2 T3
Figure 3 Mean standard error of the mean (SEM) of the
normalised melatonin concentrations at T1, T2 and T3 for
Nights 1 and 2. Melatonin concentrations at T2 and T3
were normalised to the concentrations obtained at T1,
which was always obtained in circadian darkness because
participants were wearing the orange-tinted glasses for
1 hour prior to saliva data collection at T1. Normalised
melatonin concentrations at T3 were significantly higher
than at T2 on Night 1 (t17 ¼4.8; p50.001) but not on Night
2(t17 ¼0.12; p40.05). Normalised melatonin concentra-
tions were also significantly higher on Night 1 than on
Night 2 after 1 hour of self-luminous device use (T2) and
after 2 hours of self-luminous device use (T3) without
orange-tinted glasses (t17 ¼3.3; p¼0.004 and t17 ¼4.8;
p50.001, respectively)
Melatonin suppression in adolescents 7
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higher than the suppression observed in
Figueiro et al.,
17
in which all participants
were college students (mean age ¼28 years).
In their studies, 1-hour exposure to computer
screens delivering a CS value of 0.19 sup-
pressed melatonin by about 16%. In the
present study, 1-hour use of self-luminous
devices delivering a CS value of 0.04 was
sufficient to significantly suppress melatonin
by about 23%. Wood et al.
18
showed that
participants in their study (mean age ¼19
years) who were exposed to iPads delivering a
CS value of 0.03 for 1 hour suppressed
melatonin by about 7%. In their study, five
of the 13 participants were between 13 and
15 years of age. Consistent with the hypoth-
esis that some younger adolescents may be
more sensitive to evening light for melatonin
suppression, a post-hoc re-analysis of their
data revealed that four of these younger
participants were among the ones who exhibit
the greatest melatonin suppression after using
the iPads. Their measured melatonin suppres-
sion amounts ranged from 31% to 59% after
a 2-hour exposure to light from the iPads.
Except for one participant who suppressed
melatonin by 39%, all other older partici-
pants (ages between 18 and 29 years) exhib-
ited melatonin suppression amounts below
30% after using the iPads for 2 hours. These
results are also consistent with Higuchi
et al.,
22
who showed a significant melatonin
suppression by light (140 lux) in children
(mean age = 9 years) but not in adults (mean
age = 42 years).
It should be noted that in the present study,
participants were wearing the Daysimeter as a
pendant, unlike the studies by Figueiro et al.
17
and Wood et al.,
18
where participants were
asked to wear the device at eye level. Given
that predictions of acute melatonin suppres-
sion with Daysimeter data have been shown to
be fairly close to actual suppressions in other
studies,
17,18
the lower CS values obtained in
the present study may have been an artifact of
device location. For example, if participants
were using a cell phone close to the eye, the
pendant device will likely not capture that light
exposure; therefore, the actual light exposures
during the experiment may have been higher
than those recorded by the pendant
Daysimeter. Measurements of various types
and models of self-luminous devices performed
at our laboratory
23
revealed that certain desk-
top computer screens deliver a CS value of as
much as 0.27 (equivalent to 27% suppression).
Since in the present study, suppression after 1
hour was about 23%, it is possible that CS
values from the self-luminous devices used by
participants during the study were higher than
those recorded by the pendant Daysimeter.
This assumption would only be valid, however,
if all participants were using certain models of
desktop devices, which did not seem to be the
case in the present study. Self-reports of device
usage showed that not all of the participants
were using desktop computers. Therefore, the
present data may suggest that adolescents
are more sensitive to evening light than other
populations. Future studies, using Daysimeters
that measure light at the eye, should be
designed to specifically confirm the present
findings.
As with every field study, the present study
has limitations. Even though participants were
asked to use their self-luminous devices at the
same time on both nights of the study, there
was, in some cases, a 30 to 120 minute
difference in the data collection start and end
times. However, because the first saliva sample
was always collected after participants
remained in circadian darkness for 1 hour,
this clock time difference was taken into
account in the normalised melatonin data
analyses. It is still not possible, from the present
data set, to determine whether it was the light
from the self-luminous displays alone or in
combination with the ambient light that sup-
pressed their melatonin. Moreover, it is not
possible to determine exactly what type of self-
luminous device is more likely to result in acute
melatonin suppression because participants
8Figueiro and Overington
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were allowed to use more than one type of self-
luminous device in one night. In addition, even
though participants were asked to use the same
type of self-luminous device on both experi-
mental nights, the type of device that each
participant used (i.e. computers, tablets, e-
readers, televisions and cell phones) differed.
Nevertheless, the data strongly suggest that
adolescents may be more sensitive to evening
light for acute melatonin suppression. The
present study has face validity, given that, in
real life, one may use more than one type of
self-luminous device before bedtime.
The practical implications of these results
could be significant. If the present results are
replicated in future studies that are designed to
specifically test the hypothesis that adolescents
are more sensitive to evening light than other
populations, individuals can take immediate
actions to reduce exposure to evening short-
wavelength light from self-luminous devices.
For example, adolescents may choose to turn
off self-luminous devices about 2 hours prior to
their desired bedtimes, or if this is not possible,
they can filter out the short-wavelength light by
adding theatrical gels to their devices or by
wearing the orange-tinted glasses. These simple
actionsmayresultinearlierbedtimesthat
should help improve mood, wellbeing and
perhaps even school performance in this popu-
lation. Keep in mind, however, that simply
using these devices may be stimulating to the
brain, even if melatonin is not being suppressed.
Given that morning light (i.e. after min-
imum core body temperature) is important to
promote circadian entrainment, the use of
these self-luminous devices upon waking may
be a useful way to deliver circadian active,
entraining light to teenagers. Future work
needs to be performed, however, to further
investigate the effectiveness of light from self-
luminous displays on entraining the circadian
system and to determine whether teenagers
also show a heightened sensitivity to light in
the phase advance portion of the phase
response curve.
Funding
This study was funded by the Lighting
Research Center at Rensselaer Polytechnic
Institute.
Acknowledgements
The authors would like to thank Mark S.
Rea, Barbara Plitnick, Sharon Lesage, Dennis
Hull, Greg Ward, Andrew Bierman, Rebekah
Mullaney, Dennis Guyon and Sarah Hulse
for their technical and editorial support.
Participants are also acknowledged.
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