Liisa Kuula

University of Helsinki | HY · SleepWell Program, Research Program Unit

A body of research has developed around sleep variations and romantic love. No clear understanding of the factors which are associated with specific sleep variations in people experiencing romantic love has been developed. This study investigated possible predictors of self-reported need for less sleep since falling in love among an international s...

Objectives Evidence of the impact of chronic stress on sleep is abundant, yet experimental sleep studies with a focus on acute stress are scarce and the results are mixed. Our study aimed to fill this gap by experimentally investigating the effects of pre-sleep social stress on sleep dynamics during the subsequent night, as measured with polysomnog...

The natural world is under unprecedented and accelerating pressure. Much work on understanding resilience to local and global environmental change has, so far, focussed on ecosystems. However, understanding a system’s behaviour requires knowledge of its component parts and their interactions. Here we call for increased efforts to understand ‘biolog...

Sleep is especially important for emotional memories, although the mechanisms for prioritizing emotional content are insufficiently known. As during waking, emotional processing during sleep may be hemispherically asymmetric; right-lateralized rapid-eye movement (REM) sleep theta (~4–7 Hz) is reportedly associated with emotional memory retention. N...

This article presents a model to explain why individuals experiencing romantic love have greater depression symptoms scores. We start by defining romantic love and providing a narrative review of studies investigating depression symptoms in people experiencing romantic love. We then present data from Finnish adolescents which shows higher depressio...

Circadian rhythms relate to multiple aspects of health and wellbeing, including physical activity patterns. Susceptible circadian regulation predisposes to circadian misalignment, poor sleep, sleep deprivation, increased sleepiness, and thereby sedentary behavior. Adolescents’ circadian regulation is particularly vulnerable, and may lead to sedenta...

Study objectives The day-to-next day predictions between physical activity (PA) and sleep are not well known, although they are crucial for advancing public health by delivering valid sleep and physical activity recommendations. We used Big Data to examine cross-lagged time-series of sleep and PA over 14 days and nights. Methods Bi-directional cro...

Circadian rhythms orchestrate brain function and mental wellbeing. We compared circadian patterns derived from continuous measurements of body temperature, sleep actigraphy and self-reported circadian preference in relation to different psychiatric disorders. 342 adolescents (70% females) aged 17.4y underwent M.I.N.I. psychiatric interviews, wore I...

During sleep, memories are consolidated via oscillatory events that occur in temporal and phasic synchrony. Several studies show that sleep spindles peaking close to the depolarized positive peaks of slow oscillations (SO) associate with better retention of memories. The exact timing of this synchrony presumably depends on the properties of the rel...

The neurophysiological properties of rapid eye movement sleep (REMS) are believed to tune down stressor-related emotional responses. While prior experimental findings are controversial, evidence suggests that affective habituation is hindered if REMS is fragmented. To elucidate the topic, we evoked self-conscious negative affect in the participants...

Professional and colloquial sleep hygiene guidelines advise against evening physical activity, despite meta-analyses of laboratory studies concluding that evening exercise does not impair sleep. This study is the first to investigate the association between objectively measured evening physical activity and sleep within a real-world big-data sample...

This article provides a narrative review of what is known about romantic love and sleep variations and provides possible explanations for the association. Romantic love and sleep are described using a comprehensive, unifying framework advocated by Tinbergen. We summarise the findings of studies investigating the relationship between romantic love a...

Accumulating evidence emphasizes the relevance of oscillatory synchrony in memory consolidation during sleep. Sleep spindles promote memory retention, especially when occurring in the depolarized upstate of slow oscillation (SO). A less studied topic is the inter-spindle synchrony, i.e. the temporal overlap and phasic coherence between spindles per...

The individual variation in the circadian rhythms at the physiological level is not well understood. Albeit self-reported circadian preference profiles have been consolidated, their premises are grounded on human experience, not on physiology. We used data-driven, unsupervised time series modelling to characterize distinct profiles of the circadian...

Experience sampling studies have shown that people act out of character a lot of the time. These findings have raised the question of potential costs of counter-habitual behavior. The present experience sampling study (N = 242; measurement occasions = 4342) tested, for five behavioral dimensions derived from the Big Five theory, whether self-report...

How sleep regulates physiological stress in healthy individuals is not well understood. We explored the associations between naturally occurring pre-sleep physiological arousal and EEG power spectral density together with rapid eye movement sleep (REMS) continuity. One hundred and fifty-four individuals (mean age 16.9, SD 0.1 years) collected five...

A wealth of studies supports the role of sleep in memory performance. Experimentally controlled studies indicate that prolonged wake after memory encoding is detrimental for memory outcome whereas sleep protects from wake-time interference and promotes memory consolidation. We examined how the natural distribution of wake and sleep between encoding...

We used crowdsourcing (CS) to examine how COVID-19 lockdown affects the content of dreams and nightmares. The CS took place on the sixth week of the lockdown. Over the course of 1 week, 4,275 respondents (mean age 43, SD = 14 years) assessed their sleep, and 811 reported their dream content. Overall, respondents slept substantially more (54.2%) but...

Ecosystems are under unprecedented and accelerating pressures. Much work on understanding resilience to these pressures has, so far, focussed on the ecosystem. However, understanding a system’s behaviour also requires knowledge of its component parts and their interactions. Here we present a framework for understanding ‘biological resilience’, or t...

BACKGROUND Wearable devices are used for providing objective markers of sleep. OBJECTIVE We examined the validity of heart rate variability based Firstbeat sleep analysis method (FB) against polysomnography (PSG) in a sample of healthy adults. METHODS Twenty adults (mean age 24.5 years, standard deviation (SD) 3.5, range 20–37 years; 50% females)...

Background Polysomnography (PSG) is considered the only reliable way to distinguish between different sleep stages. Wearable devices provide objective markers of sleep; however, these devices often rely only on accelerometer data, which do not enable reliable sleep stage detection. The alteration between sleep stages correlates with changes in phys...

The interactions of the principal circadian clock with the homeostatic sleep process create the time-sensitive window for easy falling asleep in the evening, which is affected by a thermoregulatory process. It has been hypothesized that the changes in skin and core body temperatures before the sleep onset might play a direct role in sleep regulatio...

Background: Very late sleep rhythms are risks for social adjustment problems in adolescence. Using ecological momentary assessment data, we quantified and visualized temporal and contemporaneous within-persons dynamical relations of sleepiness and emotions in adolescents with and without late sleep rhythms. Methods: We analyzed a temporal network v...

Objective Adolescence is often associated with decline in physical activity (PA) and a circadian shift towards eveningness, but it is not known whether these transitions are intertwined. We explored longitudinally and in cross-section how chronotype and genetic liability for morningness associate with PA as self-reported and measured by actigraphy...

Knowledge on efficient ways to reduce presleep arousal and, therefore, improve sleep, is scanty. We explored the effects of presleep slow breathing and music listening conditions on sleep quality and EEG power spectral density in young adults in a randomized, controlled trial with a crossover design. Participants’ (N = 20, 50% females) sleep was me...

Chronotype is the temporal preference for activity and sleep during the 24 h day and is linked to mental and physical health, quality of life, and mortality. Later chronotypes, so-called “night owls”, consistently display poorer health outcomes than “larks”. Previous studies have suggested that preterm birth (<37 weeks of gestation) is associated w...

Objective Early-stage romantic involvement may resemble hypomania in its manifestation on behavioral, physiological, and psychological levels. Previous research suggests that self-reported sleep duration may diminish as a result of falling in love during adolescence. We investigated how feelings of infatuation are related to subjective and objectiv...

Sleep spindles are thalamocortical oscillations that contribute to sleep maintenance and sleep-related brain plasticity. The current study is an explorative study of the circadian dynamics of sleep spindles in relation to a polygenic score (PGS) for circadian preference towards morningness. The participants represent the 17-year follow-up of a birt...

Background Depression even at the subclinical level is often accompanied by sleep disturbances, but little is known about the dynamics of the sleep stages in relation to depressive symptoms. We examined whether the amount, associations, and transition probabilities of various sleep stages were associated with depressive symptoms in a community samp...

Introduction Delayed Sleep Phase (DSP) is an extremely common sleep problem during adolescence. It is typically manifested as frequently occurring late bedtimes and difficulties waking up. The prevalence in entire populations is estimated to be over 15%. While it is common in typically developing teenagers (estimates ranging from 15%-50%), its prev...

Background: Development induces changes in sleep, and its duration has been reported to change as a function of aging. Additionally, sleep timing is a marker of pubertal maturation, where during adolescence, the circadian rhythm shifts later. Typically, this is manifested in a later sleep onset in the evening and later awakening in the morning. Th...

A common single nucleotide polymorphism (SNP) of the brain-derived neurotrophic factor (BDNF) gene, Val66Met, has been reported to impair BDNF secretion and memory function. However, few studies have investigated the interaction of BDNF genotype and sleep characteristics, such as sleep spindles, that promote long-term potentiation during sleep. In...

Study objectives: Autistic traits present a continuum from mild symptoms to severe disorder and have been associated with a high prevalence of sleep problems. Sleep spindles have a key function in sleep maintenance and in brain plasticity. Previous studies have found decreased spindle activity in clinical autism. Here we examine the associations b...

Sleep spindles are EEG oscillations in NREM sleep which are implicated in sleep-related cerebral plasticity. Several studies indicate that sleep spindle characteristics are correlates of general cognitive ability with a possible sexual dimorphism, but most of the literature consists of small studies with heterogeneous results. In order to clarify t...

ADHD and its subclinical symptoms have been associated with both disturbed sleep and weakened overnight memory consolidation. As sleep spindle activity during NREM sleep plays a key role in both sleep maintenance and memory consolidation, we examined the association between subclinical ADHD symptoms and sleep spindle activity. Furthermore, we hypot...

Background: Sleep facilitates the extraction of semantic regularities amongst newly encoded memories, which may also lead to increased false memories. We investigated sleep stage proportions and sleep spindles in the recollection of adolescents' false memories, and their potential sex-specific differences. Methods: 196 adolescents (mean age 16.9...

Objective: To determine whether the main reason for bedtime is associated with sleep and adaptive functioning in adolescents. Methods: Participants were 1374 adolescents (X age = 16.8 years, SD = 0.58; 33.6% male) from Helsinki, Finland. Adolescents completed a questionnaire battery including the Munich Chronotype Questionnaire, Strengths and Di...

Introduction Fragmented REM sleep may impede overnight resolution of distress and increase depressive symptoms. Furthermore, both fragmented REM and depressive symptoms may share a common genetic factor. We explored the associations between REM sleep fragmentation, depressive symptoms, and a polygenic risk score (PRS) for depression among adolescen...

Objective: Diagnosed autism spectrum disorders have been associated with a high prevalence of sleep problems, other psychiatric disorders and social deficits in adolescence. However, little is known about the possible connection between subclinical autistic traits and sleep. This study explored whether adolescents with elevated levels of subclinic...

Schizophrenia has been associated with disturbed sleep, even before the onset of the disorder, and also in non‐schizophrenic first‐order relatives. This may point to an underlying genetic influence. Here we examine whether weighted polygenic risk scores (PRS) for schizophrenia are associated with sleep spindle activity in healthy adolescents. Our s...

Objective: Recent genome-wide association studies (GWASs) have revealed new genetic variants behind self-reported individual circadian preference, a distinct biological trait that is fairly stable during adulthood. In this study we analyze whether these genetic variants associate with objectively measured sleep timing from childhood to adolescence...

Study objectives: The validity of consumer-targeted wrist-worn sleep measurement systems has been little studied in children and adolescents. We examined the validity of a new fitness tracker (PFT) manufactured by Polar Electro Oy and the previously validated Actiwatch 2 (AW2) from Philips Respironics against polysomnography (PSG) in children and...

A preference for eveningness (being a “night owl”) and preterm birth (<37 weeks of gestation) are associated with similar adversities, such as elevated blood pressure, impaired glucose regulation, poorer physical fitness, and lower mood. Yet, it remains unclear if and how preterm birth is associated with circadian preference. The aim of this study...

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Objectives: To assess differences relating to circadian preference in objectively measured sleep patterns from childhood to adolescence over a 9-year period. We hypothesized there is developmental continuity in sleep timing and duration according to circadian preference. Study design: Young participants (N = 111, 65% girls) from a community-base...

Individual circadian preference types and sleep EEG patterns related to spindle characteristics, have both been associated with similar cognitive and mental health phenotypes. However, no previous study has examined whether sleep spindles would differ by circadian preference. Here, we explore if spindle amplitude, density, duration or intensity dif...

Experimental sleep deprivation studies suggest that insufficient sleep and circadian misalignment associates with poorer executive function. It is not known whether this association translates to naturally occurring sleep patterns. A total of 512 of full-term-born members of the Arvo Ylppö Longitudinal Study [mean age = 25.3, standard deviation (SD...

Research suggests an association between schizophrenia and a decrease in sleep spindle activity, as well as a change in sleep architecture. It is unknown how the continuum of psychotic symptoms relates to different features in the sleep electroencephalogram. We set out to examine how sleep architecture and stage 2 spindle activity are associated wi...

Earlier puberty, especially in girls, is associated with physical and mental disorders. Prenatal glucocorticoid exposure influences the timing of puberty in animal models, but the human relevance of those findings is unknown. We studied whether voluntary consumption of licorice, which contains glycyrrhizin (a potent inhibitor of placental 11β- hydr...

Circadian preference toward eveningness has been associated with increased risk for mental health problems both in early adolescence and in adulthood. However, in late adolescence, when circadian rhythm naturally shifts to later, its significance for mental health is not clear. Accordingly, we studied how circadian rhythm estimated both by self-rep...

Objectives: To assess the longitudinal effects of sleep duration and quality on lipid profiles during the transition from childhood to early adolescence, over a 4-year-period. Study design: A cohort study of children born in 1998 examined at 8 years of age (SD, 0.3; n = 105) and 12 years of age (SD, 0.5; n = 190). Sleep duration, wake after slee...

Background: Evidence regarding the associations between sleep duration and quality, and neurocognitive function in adolescents remains scanty. This study examined the associations in early adolescence between: sleep duration; efficiency; fragmentation; wake-after-sleep-onset (WASO); catch-up sleep; intelligence; memory; and executive function, incl...