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He recently spoke to �Թϱ��� about how humans have evolved alongside their sleeping environments, the importance of sleep to our physical and emotional states and shared some practical tips for getting a good night’s rest.

What are the benefits of getting a good night’s sleep, and on the flipside, not having it?

If you get a proper amount of sleep, it will regulate your immune function, your social function, your ability to process energy and metabolism effectively. In the past five years, the linkage between mental health and sleep has been very clearly demonstrated. One massive randomized control trial that looked at psychological intervention for mental health problems actually highlights sleep as one of the core answers to ameliorate several different mental disorders. In particular, insomnia was a causal factor for the occurrence of psychotic experiences.

University students are particularly susceptible to this because a lot of university students will forgo sleep for a myriad of reasons. There has definitely been a link between nighttime sleep duration and its frequency, and psychological distress. One of the best ways university students can buttress their mental health is by focusing on their sleep quality.

This is because REM in particular is implicated in the function of emotional regulation. Say something bad happens, like you’re going through a breakup. One of the only ways to remove the emotional effect – that signal of emotional feeling that your brain links to the memory – is through REM sleep. So, when your mom says, “Sleep on it, you’ll feel better in the morning,” there is science to back that up.

We’re often told you should sleep eight hours each night. Is this true?

The National Sleep Foundation recommends seven to nine hours. In my research, I couldn’t find anything that showed me eight was a magic number for humans. In fact, when I went to look at the Hadza, small-scale hunter-gatherers in Tanzania, coupled with what the reports actually were for how much people were sleeping in the West and the global North, I think a more appropriate average is seven hours. You’re not going to be in an increased risk group if you’re sleeping between seven and nine. On the other hand, if you sleep too long, this is also linked with depressive disorders, and you’re twice as likely to die of all kinds of causes of mortality if you sleep double digits on average.

I encourage my students to not eat for three hours before going to bed because every distinct organ in your body has its own circadian clock, so the more you synchronize all of them, the easier it is to fall asleep. One of the things I do is focus on my circadian rhythm. I reserve vices like drinking booze or playing video games for the daytime, when your body is most prepared to be able to process and handle that.

How did you start looking at the role of sleep in human evolution?

When I first got into scientific research in graduate school, I was interested in chimpanzees and great apes in particular. Through those studies, I realized that we really didn’t know much about great ape sleep besides the fact that apes are really unique in that they – and I would include humans as apes as well – universally build sleeping platforms. Most monkeys, even large-brain and large-bodied monkeys, don’t do this. Chimpanzees, bonobos and humans are relatively unique in that they care about the kinds of sleep environments that they build and sleep in.

This got me thinking about the role of sleep in evolution, particularly from a cognitive perspective. When I went to do my post-doc at Duke University and teamed up with my adviser Professor Charles Nunn, we ended up doing some pretty sophisticated phylogenetic analyses, where essentially you’re controlling for the degree of evolutionary relatedness among species. We discovered that humans are actually quite bizarre. They’re unique when it comes to sleep characteristics: the distribution of REM, non-REM and total sleep time throughout the sleep period. Even after controlling for evolutionary relatedness, we discovered that humans slept the least, yet they had the greatest proportion of REM sleep of any primate.

Why are humans able to get by with relatively less sleep?

We came up with the sleep intensity hypothesis, which is the idea that evolution and natural selection whittled away total sleep time in our lineage, and what this allowed our lineage to do was to still get really high-quality sleep because we had very enriched sleep sites. We had sleep sites that were centralized by large group sizes; we had shelter from being able to manipulate branches and animal hides in the environment; we had the controlled use of fire. These resources allowed us to create these beautiful, high-quality sleep sites that we could revisit at any point in the 24-hour period.

There are numerous predators and threats when sleeping in the African savanna, so sleeping in a group would also help and deepen sleep. Even among modern-day sleepers, people who are socially isolated have more fragmented sleep than people who are well socially connected.

This short, high-quality sleep enabled things like social bonding, increased memory consolidation by learning new skills (particularly associated with REM, information processing and consolidation). There is also research that has shown that dreaming and REM sleep actually increase innovation, and perhaps there is some kind of function in REM where dreams simulate threatening events in order to help you prepare for them.  

Having trouble sleeping? David Samson and other experts recommend the following:

• Make your sleeping environment a sacred place – create that “nest” and remove a lot of distractions
• Having a big LCD screen in front of you emitting blue wave light is a horrible idea – limit cellphone usage, or, if you do use your cellphone, use a blue light blocker
• Don’t drink caffeine in the afternoon – caffeine has an incredibly long half-life as a neural modulator, so even if you think a cup of coffee isn’t affecting you, it is still in your neuroendocrine system
• Train your brain – having a nightly routine with a consistent bedtime is an important part of sleep hygiene, experts say. It’s also a good idea to avoid using your bed for lounging on during the day, reading or watching TV, so that when your head hits the pillow you know it’s time for sleep
• Keep it cool (or warm) – the temperature of your bedroom can also make a difference on how soundly you sleep; maintaining a sleep diary for a while may help you figure out what works best for you
• Tune out – bedtime can be the moment when your brain latches on to worrying thoughts. Try giving your brain something else to focus on instead of anxious thoughts. Listening to a sleepy podcast like Nothing Much Happens, , a recording of whale sounds or rainfall can help
• Daytime equals active time – experts advise against exercising late at night but moving your body more during the day can really help regulate sleep
• Don’t just lie there – if you can’t fall asleep or wake up and can’t get back to sleep, try not to dwell on your inability to sleep. Instead, consider getting up and doing something calming for a while (read, sip some water) before trying again.