Sleep. We all need it, we really notice when we don’t get enough of it, and some need more of it than others. But what is the science behind this everyday act?
You will spend around a third of your life asleep. This means if you live to be ninety years old, you will spend about thirty of those years asleep! But this is not the inactive time it might first appear to be. Scientists still don’t agree on why we need sleep; some say it is memory related while others suggest it is about clearing toxins from the brain.
All humans, mammals and most other animals need to sleep regularly to survive. In mammals and birds, sleep can be divided into two types: REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. REM-sleep usually occurs periodically throughout a night and is typically when dreams occur. As the name suggests, our eyes move rapidly during REM-sleep. NREM-sleep has four stages, the first being very light dozing, the second being light sleep, and stages three and four are deep sleep. Adult humans typically spend around half of their time asleep in light sleep.
Due to the way REM-sleep occurs periodically throughout the night, we sleep in cycles, in which we typically go through the stages of NREM-sleep, and then into REM-sleep for a while until the cycle begins again. In a full night’s sleep, four or five of these cycles may occur. Forcing ourselves to wake up in the middle of a cycle using an alarm can leave us feeling groggy and tired, whereas waking up at the end of a cycle allows us to be alert and generally feeling more refreshed. The website http://sleepyti.me/ is a useful tool for calculating when you need to go to sleep (not go to bed!) to wake up at the end of a cycle. Furthermore, if you keep the time at which you get up each day regular, it will help you sleep better - if your body knows what time it should be waking up, it can ‘prepare’ for this, and therefore allowing you to wake up faster, and be more alert.
The timing of sleep is controlled by an internal clock within our brain, known as the circadian clock. Humans can also affect the time at which they sleep through behaviour - most of us can force ourselves not to sleep, although generally can’t force ourselves to sleep. The circadian clock has a just over 24 hour cycle, and causes rhythmic increases and decreases in body temperature. The circadian clock also causes increased release of melatonin at night. This clock influences the time at which sleep will be most effective, meaning that for most of us it is most beneficial to sleep at night, since this is when our circadian clock will cause our body temperature to be lowest. The clock’s cycle is heavily influenced by light, since this is the main indicator of time. Indeed, scientists have discovered short pulses of light (blue light being the most effective) at a specific time during the cycle can reset the clock. Modern humans often find themselves out of sync with the internal clock, due to night shifts, the widespread nature of indoor lighting and long-distance travel. If we become desynced like this, we can have difficulty sleeping even when we need to. This desynchronisation is a major contributing factor to the phenomenon known as jet lag.
In general, adults need over eight hours of sleep a day, and children and teenagers are recommended to get nine or ten hours. The time at which you can fall asleep best varies, although scientists have noticed a tendency for teenagers to prefer going to bed and waking up later. The reasons for this trend are still unclear. What we do know however, is that it is very important to get enough high quality sleep. Sleep quality can be improved by avoiding eating before bed as, after a meal, the body is engaged in digestion. Stimulants like coffee can keep us awake for longer, but can also cause us to have poor quality sleep when we do sleep. Stress can also contribute to poor sleep.
If you do lose sleep, you will go into what is termed sleep debt, which needs to be paid back like any other debt. If you lose a large amount of sleep in a short amount of time, it can be paid back by getting an extra hour or two of sleep each night. Furthermore, you can pay off long term sleep debt you may have accumulated over a period of a few weeks by going to bed at a set time, and allowing yourself to wake naturally, rather than using an alarm.
The amount of sleep, and the depth with which you take it, varies greatly from person to person. Personally, I’m a very heavy sleeper. I once slept through a storm that woke the rest of my family, and I’m told most of the rest of the street. As a consequence, I can usually be perfectly alert on only seven hours sleep, and indeed generally naturally wake up after sleeping for that long. Additionally, unlike many other people, I can’t nap - once I’m up, I’m up. In contrast to me, some people can be very light sleepers, woken by the slightest noise or movement. Scientists still don’t yet know what causes these variations.
Overall, in some ways, we know more about the surface of Mars than we do about sleep! We still don’t know exactly why we need it, what all of its functions are, or why it varies for different people. All we know for fairly certain is its importance, and how we can improve our sleep quality. We also know that quality is more important than quantity when it comes to sleep, just like an essay. Just as we continue to discover more about the world around us, whether that be the depths of our oceans or the far reaches of space, scientists will continue to investigate the mechanics and reasons for sleep. In the meantime, sleep will continue to be a generally unnoticed part of (almost) everyone’s day, an ever-present but complex process.