Circadian rhythms and teenagers are often misunderstood. As we will find out, there are both biological and environmental factors at play. To help teenagers be at their best, we need to understand how these are related. We will examine puberty and sleep before we dive into the six most powerful circadian strategies to optimise teenage health.

Puberty

Puberty is a critical transitional developmental period during which extensive physical, hormonal, neural, and behavioral changes occur. These changes are essential for entering adulthood with sexual and reproductive maturity and cognitive and emotional independence. It is a period of intense cerebral neuroplasticity, finely tuned by numerous factors working in concert to enable proper brain maturation [1].

These changes mirror complex modifications within the central nervous system, including within the hypothalamus. These modifications result in the maturation of a fully active hypothalamic–pituitary–gonadal (HPG) axis [1]. The tempo of puberty is genetically determined but is also sensitive to numerous modifiers, from metabolic and sex steroid signals to environmental factors, for example, light [2].

Melatonin and puberty

Melatonin is an inhibitory neurotransmitter for puberty onset. It is very high in prepuberty boys and sharply declines (75% of its level) with the onset of puberty [3]. It is believed that this decline allows the surge of the hypothalamic gonadotropin-releasing hormone (GnRH) and the start of the cascade of pubertal changes. Melatonin is inversely correlated with testosterone (sexual maturation) and follicle-stimulating hormone (FSH, reproduction) [4]. Very low levels of melatonin are found in precocious puberty [5].

It stands to reason that the rise in precocious puberty might partially be driven by the exponential increase in light exposure and screen time at night, which is suppressing pineal melatonin release. Obviously, other factors such as neuroendocrine-disrupting chemicals/toxins, gonadal/adrenal tumours or exposure to exogenous hormones are also playing a role.  

As the onset of puberty happens earlier and much more frequently, an interesting consideration is whether long-term use of melatonin supplements can affect the timing of puberty [6].

Teenagers and Sleep

We have two mechanisms underlying the changes teenagers go through regarding their sleep:

1. less sleep pressure/drive (homeostatic)
2. increased sensitivity to evening light (circadian)

Major changes in sleep patterns and architecture occur during adolescence that parallel changes in reproductive and growth hormones. The changes in sleep/wake patterns during puberty include a tendency for later bedtimes, shorter sleep duration, irregular sleep schedules (sleeping longer during weekends compared to weekdays), and a greater ability to maintain wakefulness. This indicates a tolerance to the homeostatic drive for sleep (or “sleep pressure”) [7]. More mature adolescents can tolerate somewhat longer waking episodes than prepubertal adolescents [8, 9].

The maximum sleep delay occurs between the ages of 15 and 21. Girls usually peak earlier than boys due to younger pubertal onset [10]. Delayed timing of sleep during human adolescence is likely to represent a developmental change common across mammalian species. Evidence for a delayed circadian phase during puberty exists in many animal species [11]. 

The peak phase delay in humans occurs either during or following the final stages of sexual development. With an ability to adjust our own light cycles, our behavioural habits can easily alter the “timing input” of light and thus lead to a delayed circadian phase [12]. Numerous studies suggest that light stimulation influences HPG axis function [13]. Homeostatic and circadian regulation of sleep are sensitive to gonadal hormones. Gonadal hormones can alter key aspects of the master clock in the brain (suprachiasmatic nucleus [SCN]), including those necessary for circadian rhythm generation, entrainment, and coupling [14, 15, 16, 17]. Thus delayed circadian rhythms in teenagers are to be expected, yet there is a caveat.

Teens seem to have a blunted response to light exposure in the morning (phase-advance; it is hard for them to wake up as they need more and brighter light). And teenagers have an exaggerated response to light exposure in the evening (phase-delay; light at night keeps them awake more easily than adults) [11].

The less developed puberty is, the more sensitive teenagers are to light at night. Pre/mid-puberty teens show greater melatonin suppression at all tested light levels in the evening. The circadian system of pre/mid-pubertal adolescents appears to be very sensitive to light (even 15lux is sufficient to suppress melatonin in this age group) [18]. For older adolescents, the delayed timing is likely due to an increase in light exposure rather than an increase in light sensitivity [18].

Pre-bedtime use of light-emitting devices acutely suppresses melatonin production, shifts rhythms later, and causes heightened arousal in adults and teens alike [19, 20]. Unfortunately, the adverse effects of light at (LAN) night are far greater in teens than adults due to their increased sensitivity to LAN. Younger adolescents (<13.3 years) are more likely to be affected by the negative consequences of device use on total sleep time when compared to older adolescents (13.3–17 years) [21]. The highest preference to be awake late (eveningness) occurs at the age of 19.5 years for young women and 21 years for young men. From there on, the preference to be awake late declines and shifts toward a tendency to be awake in the morning (morningness) [22].

In short, teenagers have a slower accumulation of sleep drive in response to sleep deprivation and an internal clock that interprets environmental time cues differently from adults [11].

What can be done?

As we have seen, the physical, hormonal, neuronal, and behavioral changes teenagers undergo, allow them to be awake for longer and alter their sensitivity to the main circadian zeitgeber: light [11].

These developmental changes are entirely at odds with early school start times, leading to various adverse effects due to sleep deprivation and circadian disruption. As teenagers are more likely to be active late in the evening, the disruption of circadian rhythmicity and sleep increases. Grades suffer, car accidents increase, and mood issues like depression become more prominent [23, 24, 25, 26].

Here are the six most impactful strategies to address the clash of physiological, circadian and social needs:

1. Exposure to natural light in the morning

Upon waking, exposure to natural light outdoors is essential. As teenagers have a blunted response to morning light, they need the most potent light cue to reset their circadian rhythm [11, 27]. The lux level (to wake up) and color temperature (to reset circadian rhythms to the local time) need to be from natural light. To spike cortisol and shut off melatonin after waking, we need high lux levels (10’s of thousands of lux from sunlight vs 100’s of lux from artificial light). To obtain the correct timing information, we need the proper spectral distribution of light (1700K at sunrise vs 6500K from LED devices).

Teenagers will shift even later into the night without the right light cue in the morning, exacerbating the problem [28, 29].

Protocol: Minimum of 10min exposure to natural light outdoors upon waking, without glasses or contact lenses

2. Increase in natural light exposure

An increase in daytime light exposure supports teenagers to go to bed earlier and wake up earlier [30]. This behaviour promotes circadian rhythmicity and improves mood, sleep and performance. Again, you want exposure to natural light, as artificial light’s lux levels and spectral composition do not meet our biological needs.

Protocol:Have a 5-10min light and movement break outside every couple of hours

3. Block and minimise artificial light at night

The increased sensitivity of teenagers to light at night makes it essential to minimise and block artificial light sources [27, 30]. Brightness (lux levels) and wavelengths of light are important to manage to avoid sleep and circadian disruption. The dimmer the light source, the better. When it comes to wavelengths, blue light needs to be avoided, followed by green. Wearing blue light-blocking glasses is the foundation for implementing a healthy light hygiene after dark [31].

A good light hygiene can be achieved by various means:

• dimming lights and screens
• using circadian-friendly lighting
• using screen filters/software on devices
• wearing blue blockers after dark

Protocol: Decrease the brightness of light sources and block blue light entering the eyes after dark

4. Increase physical activity

More physical activity will increase the sleep drive of adults and teenagers alike. The more active we are, the more adenosine will build up as a byproduct of energy production. An increase in sleep pressure will mitigate the teenage tolerance to resist sleep and improve sleep quality [32, 33]. Physical activity will also positively impact mental and overall health [34]. 

Protocol: Incorporate 30min or more of daily exercise

5. Keep the same bed and wake times

Circadian, sleep and hormonal disruption occur when we shift bed and wake times. The all-too-common practice of sleeping in on the weekends (aka social jet lag) is leading to a cascade of health problems in the general public and teenagers. Weight gain, metabolic issues, increased inflammation, poorer eating habits and mood are but a few problems associated with social jet lag [35, 36, 37, 38, 39].

Keeping regular bed and wake times as much as possible is non-negotiable to avoid sleep, hormonal and health problems.

Protocol: Keep the same bed and wake times on school days and at the weekend

6. Enable more appropriate school start times

Adjustments ought to be made for teens and adolescents to allow them to be in harmony with the delay in their circadian rhythm and sleep drive. Adjusted (later) school start times increase sleep duration and rhythmicity, improve academic performance and reduce food addictions by 30% [40]. Naturally, the majority of teenagers prefer later school start times. 

Sleep and health problems improve when delaying school start times [41]. For policymakers, teachers and parents, such results provide a clear mandate. Later school start times decrease sleep deprivation, absence from school and drop our rates. [42]. 

That being said, it is important to note that the sleep patterns of teenagers can effectively be shifted earlier. Sleep and light hygiene routines, incrementally moving the bedtime earlier and adding morning bright light exposure are effective in helping teenagers fall asleep earlier and sleep longer [43].

Summary

Adolescents undergo major physiological changes that drive them to go to bed and wake up later than at any other age. This is natural but is also subject to altered sensitivity to light. To minimise the cascade of adverse health effects as well as poorer academic attendance and performance, a two-pronged approach makes sense.

1. As detailed above, educate and support teenagers to implement light, sleep and activity routines. 
2. Appeal to local decision-makers to move school start times for teenagers later. 

Let’s not forget that increased sunlight exposure is crucial to more resilience [27]. Combine that with more darkness at night, and we have taken massive strides towards improved circadian and overall health for teenagers and all other age groups [31, 44].

Mind your rhythm, mind your light!

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