Tuesday, 13 October 2015

Why do Young People Become 'Night Owls'? Screen Time vs Puberty

Many of us have terrible sleeping patterns, often as a result of staying up late for assignments, parties or just not realising it’s so late. This may have been especially true throughout fresher’s week and the first few weeks of lectures at university. It can be very easy to get into this habit, and it only takes a couple of late nights in a row to really throw your body clock out of touch with the rest of the world!

This is a problem that is very concerning to those who are aware, however very few people are aware of the current lack of sleep being attained by many adolescents. To be clear, I refer to adolescents as those who have started/ gone through puberty but are not yet full adults (i.e. 14-21 year olds).

One often cited study found that over 45% of adolescents in the USA have inadequate sleep (less than the recommended 8.5-9.5h per day on a regular basis). This lead to 15-52% of students reporting excessive sleepiness, frequent daytime sleepiness, oversleeping and sleeping during class, depending on the study. That’s pretty bad, but what are the consequences of a lack of sleep?

First, those with high scores for sleep deprivation were more likely to report feeling sad, depressed and there are associations with suicidality. Some studies indicate that over 70% of those with high sleep deprivation scores also reported feelings of sadness, depression and even suicidality. It has also been shown that it leads to impaired concentration. One study found that even one night with no sleep in a fully rested adolescent lead to significantly reduced cognitive processing speed and increased reaction time responses. It may not, therefore, be a surprise that improving sleep by delaying school start times by 1 hour significantly improved mood, tardiness, academic performance (especially in morning classes) and also led to a 16 % reduction in adolescent car crash rates within one year.

If that isn’t a good enough reason to sleep better. There is also evidence to suggest that getting more sleep could help prevent obesity in adolescents. This finding is still present even when most confounding variables are accounted for, including caloric intake and physical activity (although lack of sleep had an unhealthy impact on both of those). In a best cause scenario (increasing sleep from 7.5 hours to 10 hours) this could reduce the proportion of adolescents who are overweight by 4% (or 500,000 Americans).

Circadian Rhythms

Aside from social and work obligations, there are also biological reasons why you will want to sleep and eat at the right times. We have specialised cells that act as a network of body clocks in the brain, these are tuned to the day/night cycle and control our hormone levels throughout the day. See my previous post for more information, but essentially this bundle of cells called the suprachiasmatic nucleus receives input from a specialised cell in the retina that detects ‘blue light’ which indicates day time. The more blue light, the brighter the daytime it is and stops us from producing the sleep hormone melatonin, this makes sure that we start feeling sleepy and alert at the right time. These not only ensure that we are alert and sleepy at the right times, but they also prepare our organs for the right functions at the right time.

These rhythms are entrained by the day/night cycle but are endogenously generated, as removing the day/night cycle has little effect on daily activities. In the case of humans, our natural circadian rhythm (Latin for approximately daily) is about 24.12 hours in the absence of a day/ night cycle, pretty good if you ask me. Despite this, adolescents experience a shift towards a ‘night owl’ like sleeping pattern (or chronotype), so why is that?

Many people would attribute this change in sleeping patterns to lifestyle; staying up and playing video games or mindlessly browsing the internet, but this does not explain it entirely and teens have been staying up late for generations (although it has gotten worse in recent years). This means that, while increased screen time before bed is probably worsening the situation, there are more physiological reasons as well.

In the scientific community, the role of biology in this problem is being acknowledged more and more. These pass times may just be ways of occupying the time while we wait to get sleep (although engaging in stimulating activities such as video games clearly won’t help!). However, this phenomenon is seen cross-culturally, so lifestyle cannot be the only answer. So what does science have to say about this?

The phenomenon of delayed circadian phase (later bed times) is seen cross-culturally and cross-species. It has been shown that this occurs in every country measured so far (16 countries in 6 continents from pre-industrial to modern).

Figure 1 Regardless of region or socioeconomic status, adolescents gradually go to bed later and later, leading to inadequate sleep for multiple years during the weekday.

In laboratory conditions, where schedules are regulated in a way that allows for plentiful sleep, this shift towards a later bed time is still seen (typical delay between 1-3hrs). A similar delay is seen in Rhesus monkeys, rodents and many other animals. 

Puberty, sex hormones and sleep

As with most problems that teenagers go through, it all starts with puberty.  As previously mentioned, the circadian rhythm relies on hormones to exert its effects. Hormones are wonderfully complicated things, especially when it comes to puberty. What we do know, is that the shift in bed time or circadian phase nicely coincides with puberty across many species. Although there are problems with translating findings to do with puberty from rapidly developing animals like rodents to humans, that this is found in so many species (from amphibians and fish through to apes and humans) suggests it is a fairly important factor. Also, in species where puberty onset differs between genders, so is the delay in circadian phase (in humans girls go through puberty first and the phase delay also occurs in girls first).

We also know that removal of the sex organs in rodents removes this shift, and that restoring the levels in sex hormones induces the circadian phase shift. But let’s not dwell on that thought for too long. Sex hormones (well actually just oestrogen, as testosterone is converted to oestrogen before entering the brain) have a huge impact on the development of the brain throughout adolescence, which undergoes huge changes in order to become a fully adult brain (which I hope to cover one day). It would make sense that these changes also occur in the suprachiasmatic nucleus (where circadian rhythms are controlled in the brain) which would lead to altered circadian rhythms.

Mechanistically, the field is less certain about how this happens, as with much of neuroscience. One possibility could be changes in how sensitivity we are to light during the evenings and the mornings at that age. It has been shown that adolescents are more sensitive to evening light and less sensitive to morning light.  The most convincing evidence for this comes from a study that tried to use morning light exposure to try and shift the circadian phase back towards a more regular schedule. The study found no evidence that this was possible, even at the highest light intensity. The focus of this study was to study the phenomenon of ‘weekend social jet lag’ in adolescents and young adults. Typically, during the week adolescents follow a schedule in which they stay up late, but then wake up early for school, producing chronic sleep restriction. On the weekends, when they have the opportunity to recover sleep, they continue to stay up late, exposing themselves to evening light, and then they sleep in and lose several hours of exposure to morning light. This weekend light schedule drives their circadian rhythms to become even more delayed, so that by Monday morning they are completely out of synchrony with the school schedule and the cycle continues.

This highlights the impact that light from electronics during the evenings may be having on adolescent and adult sleeping patterns. By enhancing our exposure to light during the evenings, our brains don’t realise that it is late and so we stay up late and shift our sleep cycles back. We then miss out on light exposure during the mornings because we slept in and our sleep cycle remains shifted.

Why does this shift occur in the first place?

It’s all very well and good saying that this happens, it’s quite easy to see for yourself that young people stay up late. But now that we know what happens and a little but about how, why does this happen at all?

Because it is seen in so many species; it’s reasonable to assume to sort of evolutionary purpose, some sort of mechanism that enhances the chances of survival for an adolescent. This is where we start having to become a little more speculative, as is often the case when trying to explain the evolution of a behaviour in humans. In many fish and amphibians, juveniles and adolescents have a diurnal circadian rhythm (active during the day) whereas the adults are nocturnal (active at night in case you didn’t know…). This is likely to be because they eat different foods and the best time for eating them differ between adults and adolescents. It also means that the youngsters spend less time around the adults, which can have the propensity to eat their young in times or hardship!

Heading up the evolutionary tree, mammals are much less likely to resort to cannibalism, but the theme of avoiding adult contact remains. In rodents and monkeys, the adolescents often stay up later and rise later than the more dominant adults in order to ensure that they are feeding/ foraging at different times of the day. It is often the case that adults will see the adolescents as competition and will feel the need to assert their dominance.  It is possible that this is conserved in humans to some extent and adolescents stay up later than their parents in order to carve out some time for themselves, outside of the control of adults.

What can be done about it?

Now you know the what, the how and possibly the why. But what can be done to improve the amount of sleep that adolescents get? One way might be to move bed times earlier. This could work a little, but only small (about 15 minutes per night) gains have been found. Another method that hasn’t really translated into improved sleep are education programs. While knowledge about sleep hygiene can be improved by these programs, they rarely lead to real world changes, which may be due to the method of delivery, content and lack of engagement and recognition of the problem by the adolescents or any other reason you can think of.

What does seem to work is allowing adolescents to recover at the weekends, and many adolescents do this by themselves. Another important factor is changing school start times. One study (mentioned previously) found that delaying school start time by half an hour (to 08:30) increased average sleep duration by 45 minutes, reduced the number of students getting less than 7 hours sleep by 79.4%  and almost quadrupled the amount of students getting at least 8 hours of sleep per night. This significantly improved mood, fatigue, attendance and grades.

Another study, this time in US military recruits undergoing Basic Combat Training, had two groups; one with a customary sleep regimen (20:30 to 04:30) while the other had a delayed sleep regimen (23:00 to 07:00). The delayed sleep group gained approximately 31mins extra sleep/night on average, which resulted in significant improvements marksmanship, mood and were 2.3x less likely to report occupationally significant fatigue (ie fatigue that would affect their performance).

While no studies so far have measured whether reducing electronic screen time before bed actually leads to improved sleep duration, it makes a lot of logical sense. By exposing ourselves to less light at night time, our brain is more able to recognise that it is night time and tell itself to go to bed.  A lot of the time, I find myself on my phone/laptop at a normal time and then don’t realise how late it has gotten. To help with this I found an app for my phone and laptop that filters out blue light (unnatural to be seen in the evening) and dims the screen in accordance with sunrise and sunset in your area. It takes about 2 minutes to set up and, anecdotally at least, it seems to work (oh and they’re both free and have no ads so what do you stand to lose?). 


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