Acute Sleep Responses in a Normobaric
, GREGORY WHYTE
, STEPHEN EMEGBO
, NEIL STANLEY
, IAN HINDMARCH
English Institute of Sport, St. Mary’s College High Performance Centre, Twickenham, UNITED KINGDOM;
Medical Institute, Northwick Park Hospital, Harrow, UNITED KINGDOM;
HPRU Medical Research Centre, School of
Biomedical and Molecular Sciences University of Surrey, Guildford, UNITED KINGDOM; and
Sports Sciences, Brunel
University, Uxbridge, UNITED KINGDOM
PEDLAR, C., G. WHYTE, S. EMEGBO, N. STANLEY, I. HINDMARCH, and R. GODFREY. Acute Sleep Responses in a
Normobaric Hypoxic Tent. Med. Sci. Sports Exerc., Vol. 37, No. 6, pp. 1075–1079, 2005. Purpose: Sleeping in a hypoxic environment
is becoming increasingly popular among athletes attempting to simulate a “live high, train low” training regime. The purpose of this
study was to investigate the acute effects (one night) of sleeping in a normobaric hypoxic tent (NH) (PO
⫽110 mm Hg ⬇2500 m)
upon markers of sleep physiology and quality, compared with sleep in a normal ambient environment (BL) (PO
⫽159 mm Hg ⬇sea
level) and sleep in a normobaric normoxic tent (NN) (PO
⫽159 mm Hg). Methods: Eight male recreational athletes (age 34.5 ⫾6.9
yr; stature 169.1 ⫾8.7 cm; mass 69.3 ⫾8.2 kg; VO
56.4 ⫾8.3 mL䡠kg
) participated in the study using a randomized,
double-blind crossover design. Polysomnographic studies were undertaken to measure sleep stages, arterial oxygen saturation (SpO2),
heart rate (HR), and the Respiratory Disturbance Index (RDI). The Leeds Sleep Evaluation Questionnaire (LSEQ) was used to measure
subjective sleep quality. Results: NH (89.9 ⫾4.8%) resulted in a significantly lower (P⬍0.05) SpO
compared with both BL (95.7 ⫾1.5%)
and NN (93.5 ⫾4.0%). Heart rate was significantly higher (P⬍0.05) in NH (51.5 ⫾7.6 beats䡠min
) compared with NN (48.3 ⫾6.9
) but was similar versus BL (50.3 ⫾4.3 beats䡠min
). RDI (counts䡠h
) and RDI (total counts) were lowest in BL (3.5 ⫾2.5;
18.1 ⫾7.9) and highest in NH (36.8 ⫾42.7; 221.9 ⫾254.5). The difference in RDI (counts䡠h
and total counts) between NH and BL was
significant (P⬍0.05). The LSEQ revealed that subjects’ “behavior following waking” score was significantly (P⬍0.05) lower in NH (40.9
⫾9.2) compared with BL (52.3 ⫾8.3). Conclusion: This study presents evidence that sleep in a normobaric hypoxic tent at a simulated
altitude of 2500 m may affect sleep parameters in some individuals. This type of analysis may be useful in the early identification of poorly
responding individuals to simulated altitude environments. Key Words: RESPIRATORY DISTURBANCE INDEX, ALTITUDE TRAIN-
ING, POLYSOMNOGRAPHY, SIMULATED ALTITUDE, LIVE HIGH, TRAIN LOW
The physiological load placed on the human at altitude
results in physiological adaptations, primarily to the
oxygen transport system, that may be beneficial to
athletic performance (4). These adaptations are primarily
mediated by a reduced O
flux at moderate altitude (2000 –
3000 m). Maximal sustained exercise capacity is reduced,
however, and therefore training quantity and quality may be
hindered at moderate altitude (1,12). To combat this, ath-
letes often descend from altitude to perform training ses-
sions, returning to altitude at night to continue the acclima-
tization process. This concept of living high and training
low (HI-LO) has received considerable attention within the
literature. To date, no consensus on the efficacy of HI-LO
training has been reached with evidence supporting a physio-
logical and performance enhancement effect and evidence
doubting purported physiological gains (3,7,9,15,26). Never-
theless, athletic populations are increasingly using nocturnal
hypoxia in an attempt to gain some physiological benefit.
The principle methods used by athletes to achieve a
HI-LO training regime, without the complications of trav-
eling to and from true altitude (hypobaric hypoxia) include
sleeping in normobaric hypoxia while at sea level or breath-
ing a hyperoxic gas mixture while training at true altitude.
Living at sea level and sleeping in a normobaric hypoxic
environment artificially simulating an altitude of 2500 m is
a popular form of HI-LO training, with a number of facil-
ities ranging from hypoxic hotels to individual hypoxic tents
becoming increasingly popular (27).
In a recent study Kinsman and coworkers (14) attempted
to quantify the effect of normobaric hypoxia (2650 m) upon
markers of sleep quality. This study investigated respiratory
events in cyclists participating in a HI-LO program using a
nitrogen enriched facility, reporting a substantial increase in
respiratory events during sleep, present in nearly 25% of the
athletes studied. This study did not, however, investigate
sleep in a normobaric hypoxic tent, which is an alternative
method commonly used by athletes.
Address for correspondence: Charles Pedlar, English Institute of Sport, St.
Mary’s College High Performance Centre, Waldergrave Road, Twicken-
ham, TW1 4SX, United Kingdom; E-mail: email@example.com
Submitted for publication September 2004.
Accepted for publication February 2005.
MEDICINE & SCIENCE IN SPORTS & EXERCISE
Copyright © 2005 by the American College of Sports Medicine
There remains a dearth of data evaluating sleep in nor-
mobaric hypoxic tents resulting in limited practical guid-
ance for the athlete and coach regarding HI-LO programs.
Accordingly, the aim of the present study was to examine
the effects of an acute exposure to normobaric hypoxia,
simulating an altitude of 2500 m upon sleep characteristics.
Participants. Eight healthy male nonsmoking recre-
ational athletes participated in the study (age 34.5 ⫾6.9 yr;
stature 169.1 ⫾8.7 cm; mass 69.3 ⫾8.2 kg; VO
). All subjects were screened for a
period of 7 d before the study period using wristwatch
actigraphy (Cambridge Neurotechnologies, Cambridge,
UK). No subjects showed any evidence of sleep disruption
during this period. After approval by the local ethics com-
mittee, participants were fully informed of the study and
provided written informed consent.
Research design. All athletes undertook a progres-
sive cycle ergometry test to volitional exhaustion. Online
gas analysis and 12-lead electrocardiogram (Jaeger Oxycon-
Pro, Viasys Healthcare, UK) were employed to establish
and to perform a cardiovascular screening. Resting
maximal flow-volume loops were performed by each athlete
to record forced vital capacity (FVC) and forced expiratory
volume in 1 s (FEV
Subjects were subsequently investigated over three
nights. Between 2100 and 2200 h, subjects were prepared
for polysomnography, that is, placement of electrodes. All
subjects then remained in a lounge area and were free to go
to bed. All subjects were required to be in bed by 2300 h and
remain in the tent until 0700. Polysomnographic recordings
began upon going to bed in individual rooms (temperature
⫽15.5 ⫾2°C) until a wake-up call at 0700 h. Night 1 (BL)
was a normalization night and allowed for the monitoring of
sleep in the ambient environment of the bedroom, thus
providing baseline data. A double-blind cross-over research
design was employed for nights 2 and 3, where subjects
were randomly assigned to either normobaric hypoxia (NH)
⫽110 mm Hg) or normobaric normoxia (NN) (PO
159 mm Hg) conditions. The degree of hypoxia equal to a
true altitude of 2500 m was chosen because this is a typical
altitude chosen by athletes for HI-LO training (4). The
normobaric hypoxic environment was generated with a
commercially available tent and generator unit (Hypoxico
Inc., New York). The tent houses the subject and a standard
bed mattress within an enclosed environment constructed
predominantly from plastic. Gas is able to leak from the
enclosure through seams and zips. The hypoxic generator
units supply a nitrogen enriched gas mixture through a 4-m
plastic hose. The units were placed outside of the bedroom
to maintain silent conditions within the tent, delivering the
gas mixture through the hose provided with the unit. The
manufacturers report that the gas mixture flows at a rate of
. Before each night, the gas mixture was
checked with a portable oxygen analyser (Teledyne Analyt-
ical Instruments, Los Angeles) placed in the stream of flow
to ensure the correct F
was delivered to the tent. No
further testing of the equipment was undertaken. Equipment
was set up precisely as described by the product manufac-
turers in order to closely mimic the practices of an athlete.
Sleep polysomnography (PSG). Recording of the
nocturnal polysomnogram (Nicholet Ultrasom 5 Worksta-
tion, Madison, U.S.) was performed in accordance with
standard criteria (10), using four electroencephalography
(EEG) channels (c4-a1, o2-a1, c3-a2, o1-a2), two elec-
trooculography (EOG) channels (LOC, ROC), electromyo-
graphy (EMG) channel, and two-lead electrocardiography
(ECG). Somatic measures of nocturnal physiology included
inductive plethysmography, body position measurements,
left and right leg EMG of anterior tibialis, finger pulse
), oronasal airflow (airflow thermistor), and
the assessment of respiratory noise.
Sleep data were staged by an experienced sleep electro-
physiologist according to standard criteria (19). Sleep pa-
rameters in the analysis included stages of sleep and sleep
stage transitions. Time spent in sleep stages 1 and 2, and 3
and 4 (slow-wave sleep) and rapid eye movement (REM)
was recorded. Respiratory data were manually assessed by
a qualified sleep physiologist in accordance with American
Thoracic Society indications and standards for cardiopul-
monary sleep studies (2). A sleep apnea is defined as a
cessation of airflow for longer than 10 s. Sleep hypopnea is
defined as a 50% reduction in airflow for longer than 10 s
with a 3% reduction in SpO
. Respiratory disturbance index
(RDI) was used as a global measurement of respiratory
disturbance during sleep and is a sum of all apneas and
hypopneas (2). Data are presented in counts per hour
) and total counts (RDI
Oxygen saturation (SpO
) and heart rate (HR) were mea-
sured at 4-s intervals using a pulse oximeter (Ohmeda,
Herts, UK) attached to the index finger with medical tape.
Data were logged continuously for the whole night and an
arithmetical mean value was calculated. In addition, two-
lead ECG was used to confirm HR.
Subjective analysis of sleep quality was based on assess-
ment of the ease of getting to sleep (GTS), behavior fol-
lowing waking (BFW), and quality of sleep (QS). These
were measured using the Leeds Sleep Evaluation Question-
naire (18), where subjects are required to mark a series of
100-mm visuo-analog scales, indicating their present feeling
with regards to a midpoint, which represents their normal
state of mind before treatment began. Scores are represented
Statistical analysis. Statistical analysis was per-
formed using the Wilcoxon signed ranks test for nonpara-
metric data with alpha set at 0.05. Additionally, simple
bivariate correlations were performed to assess the relation-
ship between SpO
, and between RDI
height, weight, VO
, FVC, and FEV
NH resulted in a significant (P⬍0.05) reduction in
oxygen saturation (89.9 ⫾4.8%) compared with both BL
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
(95.7 ⫾1.5%) and NN (93.5 ⫾4.0%) conditions. Heart rate
was elevated in NH (51.5 ⫾7.6 beats䡠min
with BL (50.3 ⫾4.3 beats䡠min
) and NN (48.3 ⫾6.9
); however, this was only statistically significant
(P⬍0.05) between NH and NN.
No significant differences existed for sleep period total
(SPT) or sleep efficiency (SEff) between the three condi-
tions (P⬎0.05). Duration of sleep stages 3 and 4 and REM
sleep was not significantly different between conditions (P
⬎0.05); however, sleep stages 1 and 2 were significantly
different (P⬍0.05) between NN (248.7 ⫾54.4 min) and
BL conditions (208.2 ⫾51.4 min).
Respiratory disturbance index counts per hour (RDI䡠h
were lowest in BL conditions and highest in NH, and this
was significantly different between NH and BL conditions
(P⬍0.05, see Table 1). Considerable interindividual dif-
ferences were apparent for RDI per hour. A similar finding
occurred for RDI
, where the mean value was lowest in
baseline conditions and highest in hypoxia, and this was
significant between hypoxic and baseline conditions (P⬍
0.05, see Table 1). The composition of the respiratory dis-
turbance, that is, apneas and hypopneas, is displayed in
Table 1. The occurrence of sleep apneas was significantly
elevated in both NH and NN compared to BL conditions (P
⬍0.05). Sleep hypopneas were significantly increased in
NH compared with BL (P⬍0.05). The majority of apneas
and hypopneas were between 10 and 15 s in duration and
occurred during non-REM sleep stages.
Significant differences (P⬍0.05) occurred between NH
and BL conditions for behavior following waking (see Table
2). No significant differences were found between conditions
for quality of sleep or getting to sleep parameters (P⬎0.05).
No significant correlations were found between SpO
(r ⫽0.09; P⬎0.05). No significant correlations
were found between RDI
and height (r ⫽0.02; P⬎
0.05), weight (r ⫽⫺0.06), VO
(r ⫽0.46; P⬎0.05),
FVC (r ⫽0.13; P⬎0.05), or FEV
(r ⫽0.51; P⬎0.05).
The present study investigated the effect of acute normo-
baric hypoxia, specifically via the use of a commercially
available hypoxic tent, upon physiological and psychomet-
ric parameters of sleep in eight recreational athletes. Find-
ings suggest that some parameters of sleep were affected by
the hypoxic tent (NH) compared with placebo (NN) and
baseline conditions (BL).
NH resulted in a significant reduction in SpO
significant increase in RDI compared with NN; however,
despite this respiratory disturbance, there was no significant
difference in stages 3 and 4 or REM sleep between condi-
tions. This indicates that slow-wave, restorative sleep is
maintained in the normobaric hypoxic tent. The duration of
sleep stages 1 and 2 was significantly shorter in BL com-
pared with NN conditions, which may be due to familiar-
ization with the new environment of the sleep laboratory.
The score for behavior following waking represents the
mean of the LSEQ visual analog scales for questions 8 –10,
which relate to ease of waking up and getting up, clumsi-
ness, and feelings of alertness versus tiredness. This score
was significantly reduced after sleep in NH, presumably
related to respiratory disturbance or other factors associated
with NH resulting in a feeling of residual tiredness. The
practical implications of this may be that early morning
performance may be impaired by NH. This could potentially
impact on an athletes’ ability to train or compete effectively;
however, to the knowledge of the researchers, this question-
naire has not been used to investigate the effects of sleep
upon performance in athletes.
TABLE 2. Subjective sleep quality data for baseline, hypoxia, and placebo conditions (mean ⫾SD, range in parentheses).
GTS (1–100 scale, mean ⫾SD) 55.6 ⫾12.6 50.4 ⫾10.2 46.4 ⫾10.6
(37.0–72.3) (37.5–70.5) (29.0–65.0)
BFW* (1–100 scale, mean ⫾SD) 52.3 ⫾8.3 47.4 ⫾5.7 40.9 ⫾9.2 H-B
(42.4–64.8) (36.0–52.1) (26.4–51.0)
QS (1–100 scale, mean ⫾SD) 56.5 ⫾19.2 54.1 ⫾13.8 41.9 ⫾14.0
(36.0–92.0) (41.6–84.3) (18.5–64.0)
*Denotes significant difference (P⬍0.05); H-B denotes significant differences between hypoxic and baseline conditions; GTS, getting to sleep; BFW, behavior following waking; QS,
quality of sleep.
TABLE 1. Respiratory Disturbance Index (per hour and total counts), apneas, and hypopneas data for baseline, hypoxia, and placebo conditions (mean ⫾SD, range in
)3.5 ⫾2.5 8.0 ⫾12.2 36.8 ⫾42.7 H-B
(1.4–8.7) (0.4–37.3) (1.3–95.5)
*(counts) 18.1 ⫾7.9 49.1 ⫾66.9 221.9 ⫾254.5 H-B
(9–30) (3–208) (9–554)
APN* (counts) 1.9 ⫾1.6 19.9 ⫾36.3 91.3 ⫾112.9 H-B, P-B
(0–5) (0–108) (1–266)
HYP* (counts) 17.0 ⫾10.3 35.5 ⫾45.9 145.0 ⫾179.2 H-B
(0–35) (2–143) (8–459)
*Denotes significant difference (P⬍0.05) (H-B denotes significant differences between hypoxic and baseline conditions, P-B denotes significant differences between placebo and
RDI, respiratory disturbance index; APN, apnea; HYP, hypopnea; SPT, sleep period total.
SLEEP IN A NORMOBARIC HYPOXIC TENT Medicine & Science in Sports & Exercise姞
Data in the present study demonstrated wide heterogene-
ity between individual athletes in the response to hypoxia,
and this is evidenced by wide ranges and large standard
deviations in the data, particularly for RDI (counts䡠h
total counts). Other research involving athletes’ physiolog-
ical responses to hypoxia has repeatedly demonstrated wide
heterogeneity between individual data, and this is true of
sleep parameters (14,28), exercise capacity (6), and physi-
ological adaptation (11,22).
Previous studies have suggested a relationship between
anthropometrical measures and respiratory disturbance dur-
ing sleep in hypoxia. Ri-Li et al. (20) demonstrated a link
between body composition and symptoms of acute moun-
tain sickness (AMS) in men during a 24-h exposure to
hypobaric hypoxia (3658 m), showing that symptoms of
AMS were more common among obese men when com-
pared with normal controls. The present study, however,
found no relationship between the observed physiological
alterations during sleep and measurements of height and
, or lung function, which might help to
explain the findings.
Variations in sleep response have been observed at alti-
tude compared with sea level. Zielinski et al. (28) observed
increases in periodic breathing at an altitude of 3200 m with
wide individual variability in intensity. The number of
arousals and awakenings doubled at high altitude, and the
level of oxygen saturation (SpO
), was lower at high altitude
than at sea level. These findings concur with the findings of
the present study. We observed significant increases in
respiratory disturbance, particularly with increases in sleep
apneas in NH compared with BL. The cause of the rise in
sleep apneas and hypopneas in hypoxia is unclear but could
be related to the hypoxic environment of the tent. Others
have demonstrated respiratory disturbance during sleep in
normobaric hypoxia (14). Other factors relating to the tent
system could also have contributed to respiratory distur-
bance, because apneas were also raised in NN. Further
examination of the environment in the tent is warranted, for
example, temperature, humidity, and CO
The relationship between a reduced SpO
disturbance (periodic breathing) has been suggested by oth-
ers (16,23,14,17). Levine et al. (16) analyzed a model for
chemoreceptor mediated control of breathing. Although this
was a theoretical study, the author concluded that periodic
breathing may be a protective adaptation that serves to
improve oxygen delivery to the tissues. Salvaggio et al. (23),
investigating sleep at high altitude (5050 m), showed that
periodic breathing results in a slight improvement in SpO
In the present study, mean saturation levels dropped by
5.8% from baseline to hypoxia, and individual saturation
values fell below 88% in three of the eight subjects in
hypoxia, a level of desaturation considered clinically sig-
nificant in normal populations (SpO
Thoracic Society). Mean level of desaturation, however,
showed no relationship to respiratory disturbance or subjec-
tive measures of sleep quality in the present study. This
finding concurs with the findings of Kinsman et al. (14),
who indicated that SpO
may not be sensitive enough to
predict respiratory events and therefore likely sleep distur-
bance. Significant oxygen desaturation during sleep in a
normal environment, particularly during REM sleep is often
seen in patients with COPD; however, it is uncommon in
normal individuals (17). To the knowledge of the authors,
there are no data available in the literature regarding the
prevalence of oxygen desaturation during sleep among ath-
letic populations; therefore, the long-term health implica-
tions are unknown.
In addition to the reduced SpO
observed in the present
study, heart rate was elevated during sleep (3.2 ⫾2.5
difference between hypoxia and placebo con-
ditions). These findings concur with those of Kinsman et al.
(14) reporting a drop in average oxygen saturation of 6 ⫾
1% and an elevation in heart rate of 3 ⫾1 beats䡠min
elite athletes sleeping in normobaric hypoxic dormitories
simulating 2650 m. It has been suggested that one role of the
increased heart rate is to compensate for the reduced SpO
in order to maintain oxygen delivery (14). In contrast, other
studies reporting a reduced SpO
at high altitude compared
with sea level, failed to report any alterations in heart rate
during a 4-wk sojourn gradually ascending to 5050 m (23).
Further study is warranted in a larger cohort to allow a more
detailed examination of this relationship in normobaric hy-
poxic environments simulating moderate altitude.
Sleep apneas are linked with hypertension and heart dis-
ease in diseased populations (8); however, the long-term
health implications of sleep apneas in athletes are not
known. It is widely believed that the quality of rest and
recovery of the athlete between training bouts is an essential
part of the adaptation process (the restorative hypothesis of
sleep; (24)). Inadequate recovery may lead to symptoms of
overtraining and ultimately underperformance. The findings
of the present study may have significant implications for an
individual wishing to commence a HI-LO program using a
normobaric hypoxic tent, to avoid the detrimental effects of
poor sleep upon training and recovery.
It has been suggested that short duration (⬎90 min) of
exposure during the day may have an erythropoietic effect
comparable to nocturnal exposure (for a review, see (25)).
This would have the advantage of removing the possibility
of diminished sleep quality because the athlete can continue
to sleep in their normal environment; however, the data in
this area are limited. Rodriguez et al. (21) and Casas et al.
(5) demonstrated increased reticulocytes, hemoglobin, and
hematocrit after 9 and 17 d (3 and 5 h䡠d
, 4000-m and
5500-m altitude); however, the quality of the data may be
confounded by the measurement techniques used. A further
method of intermittent hypoxic exposure during the day via
a portable breathing device supplying hypoxic gas (F
12% reducing to 10% over 4 wk, 5 d·wk
for intervals of
5 min, separated by 5 min of normoxia for a total of 70 min)
has also recently been investigated, but no physiological or
performance improvements were observed (13).
Data from the present study suggest that the use of indi-
vidual athlete assessment in hypoxia before embarking upon
a HI-LO program using a normobaric hypoxic tent system
may be useful for identifying those athletes at risk of poor
Official Journal of the American College of Sports Medicine http://www.acsm-msse.org
sleep quality. It must, however, be noted that this study is
limited to acute exposure to simulated moderate altitude via
a normobaric hypoxic tent, with a small subject group.
Further work is indicated to examine the effect of repeated
exposures to this environment and the subsequent impact
upon sleep disturbance. It is not known whether symptoms
of disrupted sleep in a normobaric hypoxic tent would
diminish over a number of days of acclimatization.
The present study provides evidence that selected parame-
ters of sleep may be significantly affected by a normobaric
hypoxic tent unit and that there is a wide variation in the
individual response in recreational athletes. The evaluation of
sleep quality may be useful in identifying individuals who are
vulnerable to sleep disruption in a normobaric hypoxic tent.
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