The short and long of adolescent sleep: the unique impact of day length

Highlights

• Day length uniquely contributed to sleep.

• Short day length predicted later bedtimes, shorter sleep latency, and less sleep.

• Longer day length was associated with earlier bedtimes and longer sleep.

• Practicing good sleep hygiene was the most advantageous for sleep.

Abstract

Study objectives

Variation in day length is proposed to impact sleep, yet it is unknown whether this is above the influence of behavioural factors. Day length, sleep hygiene, and parent-set bedtime were simultaneously explored, to investigate the relative importance of each on adolescents’ sleep.

Methods

An online survey was distributed in four countries at varying latitudes/longitudes (Australia, The Netherlands, Canada, Norway).

Results

Overall, 711 (242 male; age M = 15.7 ± 1.6, range = 12–19 yrs) adolescents contributed data. Hierarchical regression analyses showed good sleep hygiene was associated with earlier bedtime, shorter sleep latency, and longer sleep (β = −0.34; −0.30; 0.32, p < 0.05, respectively). Shorter day length predicted later bedtime (β = 0.11, p = 0.009), decreased sleep latency (β = −0.21, p < 0.001), and total sleep (β = −0.14, p = 0.001). Longer day length predicted earlier bedtimes (β = −0.11, p = 0.004), and longer sleep (β = 0.10, p = 0.011).

Conclusions

Sleep hygiene had the most clinical relevance for improving sleep, thus should be considered when implementing adolescent sleep interventions, particularly as small negative effects of shorter day length may be minimised through sleep hygiene techniques.

Introduction

Adolescence is a time marked by relatively rapid physiological, biological, and hormonal changes, all of which contribute to changing sleep patterns [1], [2], [3]. As adolescents become older, their ability to postpone sleep onset increases, through a reduction in the escalation of sleep pressure, and a natural delay in their sleep timing (via a delayed 24-h circadian rhythm timing) [1]. This, in conjunction with constraints imposed on sleep through starting school can lead to restricted sleep, especially on weekdays [4], [5]. Healthy sleep is vital for adolescents' wellbeing [6], affecting their mental health, interpersonal problems [6], and school performance [7]. It is thus important to consider the extent to which contributing factors beyond adolescents' behavioural control (ie, age, gender, day length) influence their sleep, before examining factors within their control (ie, sleep hygiene, parent-set bedtime, alcohol, tobacco) in order to empower adolescents, and their caregivers, with the knowledge and tools to achieve optimal sleep health.

In terms of demographics, younger age is predictive of longer sleep in adolescents, yet effects of gender decrease by mid-adolescents [8]. Sleep hygiene is a factor under behavioural control, and comprises multiple facets that may benefit the sleep of adolescents, including consistent bedtime routines, avoidance of pre-bedtime stimulating substances (eg, caffeine) and activities (eg, technology use), or a comfortable sleep environment (eg, quiet and dark bedroom) [9], [10]. Parent-set bedtimes have recently been shown to be an important determinant of adolescents' sleep, being consistently linked with longer sleep durations (yet not sleep latency) [9], which in turn improve adolescents' daytime alertness and decreasing their fatigue [11], depression, and suicidal ideation [12]. However, the benefits of weekday parent-set bedtime on bedtime and total sleep do not occur when parents' limitations are temporarily removed (ie, weekends) [11].

However, one understudied area are geographic factors, which may influence when adolescents retire for bed, how long it takes them to fall asleep, and how much sleep they obtain. Notably, small shifts in longitude and latitude are suggested to impact bedtime [13], with a trend towards a negative relationship with longitude (ie, further East longitude and earlier bedtime), yet a positive relationship with latitude (ie, further North latitude and later bedtime) [9]. Trends in the association between longitude/latitude and adolescent sleep duration have not appeared [14], however, there is potential for broader ranges in longitude and latitude to allow for differences in sleep duration to be observed (ie, [14] measured adolescents at differences of 6°E and 19°N). Nevertheless, the number of studies into the associations between longitude and latitude with adolescents' sleep is extremely small (ie, four studies to the authors' knowledge) [13], [14], [15], [16], especially when compared to the plethora of studies investigating topical factors (eg, technology use; 67 studies with 5–17 year old participants) [17]. A recent meta-analysis showed the possibility of small relationships between latitude and longitude with adolescents' bedtimes, sleep latency, and total sleep time, yet unfortunately, these were based on only two studies, each with relatively small shifts in longitude and latitude [9]. Of note, changes in longitude and latitude result in variations in sunrise and sunset times, and hence day length. To increase our confidence that there does indeed exist a meaningful association between longitude and latitude (and therefore day length) with adolescents' sleep, more studies are needed. The aim of the present study was to assess the unique impact of day length on adolescents' bedtimes, sleep latency, and total sleep time by sampling across multiple countries with large variations in latitudes (Australia, The Netherlands, Canada, Norway), whilst simultaneously controlling for important factors under behavioural control (ie, sleep hygiene, parent-set bedtimes, demographics).