How Is Sleep Controlled?

Much is written about how sleep is similar to eating and drinking in terms of how essential it is to live. However, these other bodily functions are very much under our voluntary control. Why is it that the control of sleep is placed outside of our command? Indeed, the more we attempt to chase sleep, the more it floats out of reach.

Broadly speaking, sleep control is governed by two interacting systems – the circadian rhythm and the adenosine sleep drive.

What is the circadian rhythm?

As humans, we are very reliant on light for our activities. So we have evolved to have our peak activity levels during daylight hours and rest during the dark. This cyclical fluctuation is governed by an internal body clock known as the circadian rhythm. The name comes from the Latin word ‘circa’ for round and ‘dia’ for day. Several human bodily functions wax and wane in line with this circadian rhythm, including appetite, body temperature and blood sugar control.

The cycle length is set as just over 24 hours. Still, it is constantly corrected by external stimuli, known as ‘zeitgebers’, which in German means’ time-givers’. The main zeitgeber is, of course, light. This whole process of daily correction, known as entrainment, is so that our internal body clocks stay in time with the movement of the seasons.

How Is Sleep Controlled?

When light enters your eye, it is detected by your retina’s rod and cone cells. The light signal is sent to the brain by the optic nerves and onwards to the visual cortex of your brain for the interpretation of the image. A few fibres of the optic nerve ‘skim off’ a bit of that light signal and send it to a different part of the brain, called the suprachiasmatic nucleus, or SCN. This SCN is the internal pacemaker for the circadian rhythm.

In response to bright light, the SCN will trigger another part of the brain to produce a hormone called cortisol. This cortisol circulates in the bloodstream and puts the body into a wake phase – your alertness levels rise along with temperature and appetite. Cortisol will eventually be suppressed in the absence of bright light, and another hormone, melatonin, will be produced. This is the dominant hormone of the sleep phase.

What is the adenosine sleep drive?

The circadian rhythm does not explain the whole story about sleep control. The other system at work is called the adenosine sleep drive.
Adenosine is a chemical your body makes whenever you are awake. Adenosine accumulates; the more adenosine you have, the more you want to sleep. The only process that can rid the body of adenosine is sleeping.

The interaction of the two systems is shown below.

How Is Sleep Controlled?

The diagram above shows the typical circadian fluctuations in white. At 7 am, this individual has woken up, and now that they are awake, they will start generating adenosine, shown in yellow. Around 10 pm, the circadian rhythm started to wind down. This coincides with a high level of adenosine, and the person falls asleep. During sleep, their adenosine will reduce. When a low level of adenosine coincides with the predetermined up-kick in the circadian rhythm at 7 am – the person wakes up.

When you look at what happens when you pull an all-nighter, more understanding can be given.

How Is Sleep Controlled?

Rather than go to sleep at 11 pm, this person has stayed up, so the adenosine has continued to rise. Around 4 am, the circadian rhythm has reached its lowest point – any night shift worker will tell you this is the most challenging hour to stay awake, you feel cold, and your mental alertness grinds to a halt. But, come 7 am, despite the fact that your adenosine is still rising, you no longer feel sleepy, and this is because your circadian rhythm has kicked into action and is sustaining you throughout the day. The crash happens that evening; as soon as your circadian rhythm starts to turn down, your body has nothing to oppose the high level of adenosine sleep drive, and you drop into sleep.

Circadian Rhythm Disorders

The above process is, of course, how it’s meant to happen. But not everyone’s circadian rhythm dances to the same tune.

We’ve all heard of morning larks – those who are irritatingly perky at 7am, and night owls, who never seem to flag even when 1am rolls around. These are both normal variants of circadian phases. Across our life span, our circadian rhythm shifts. Teenagers often won’t naturally sleep until midnight or 1am, then insist on rising around 10am, long after the school bus has departed. Older adults will more naturally nod off around 8pm and seem to rise bright and early at 6am. A circadian rhythm disorder is where these normal variants get taken to extremes.

Circadian delays are when people cannot fall asleep until exceptionally late, perhaps 3am, and won’t naturally rise until 11am. Circadian advancement causes the opposite effect, when people start to become drowsy at unworkable times, such as 7pm, but then naturally rise around 4am. The issue arises from being unable to fit in with a conventional social schedule.
Shift worker syndrome is the name given to a more disorganised circadian misalignment that arises from a work schedule that is variably out of sync with the natural sleep-wake circadian cycle.

How Is Sleep Controlled?

Another well-known circadian disorder is jet lag, where an individual running to their own, healthy internal rhythm is flown to a time-zone that is not in sync, then expected to carry out all their daily activities, including working, eating and sleeping in time with this new regime.

So how do you deal with a circadian rhythm that’s out of step with the world around you? The answer is to bring it back into sync using the chief method of circadian entrainment – light.

Research has shown that bright light exposure, of 10,000 lux or more, for around 2 hours from the time you wake up can bring your circadian rhythm forward. This occurs by pushing back the time you would usually produce melatonin. So a teenager who struggles to sleep before 3am, or rises before 11am, could have bright light at breakfast time.

In the same way, older adults who fall asleep unconventionally early can expose themselves to bright light in the evening hours of sleepiness when they would rather be awake and postpone the natural production of melatonin that would otherwise begin their sleep phase.

As shift worker syndrome and jet lag often involve multiple disruptions to circadian synchrony, it can be more challenging to re-align. However, the same principles of timed light exposure with or without oral melatonin can help.

Everyone has different sleep requirements

It’s not only the timing of your circadian rhythm that changes across different ages but also the amount of sleep you need.

A newborn baby can sleep for 18 hours a day. This reduces to around 10 hours a day in a school-age child and an average of about 8 hours through most of adulthood. The sleep requirement drops again into older adulthood, requiring around 6-7 hours. The numbers are averages across the lifespan.

Individually, the amount of sleep we each need can vary. Some adults require 10 hours a night, whereas others are fine with 6 hours. This inbuilt requirement is primarily set by your genetics but also influenced to an extent by the way you spend your day – a strenuous day will generate a more significant sleep requirement. The key point is – not everyone needs the same amount of sleep. There is sometimes a fixed obsession with ‘your full eight hours’, which can lead to people either not sleeping enough or becoming unduly worried that they need to sleep more. The best way to judge if you have rested adequately is by how you feel the next day. If you wake feeling unrefreshed and want to go back to bed before lunchtime, chances are you haven’t slept enough!

How we get the required hours of sleep each day is often influenced by lifestyle and culture. Most people in the UK get all their sleep in one go overnight, with exceptions being shift workers or people with other responsibilities overnight, such as looking after young children. Looking across the globe – in the same way that humans developed to be more awake during daylight hours, if the middle of that day is so hot that you cannot be outside in the sun, then you naturally develop a ‘biphasic’ sleeping pattern – in other words, a siesta around lunchtime, then a period of sleep overnight. This is often facilitated by a brief dip in your circadian wakefulness around lunchtime. However, this tendency to nap at lunchtime might be because you are naturally a biphasic sleeper. It can also be how not getting enough sleep can manifest, with daytime napping being how you cope. This suits certain lifestyles, but in most cases, getting better sleep overnight can iron out this need to nap and help you fit in with a more conventional daily schedule.

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How Is Sleep Controlled?