Posted on 9 December 2022
In the previous article, we looked at why sleep is an essential but often overlooked part of the longevity puzzle, what actually happens in the brain and elsewhere during sleep, and what sets good sleep apart from bad. In order to set ourselves up for the longest, most efficient and most restorative night of sleep possible, we now need to understand how sleep patterns are actually controlled. How does the body ‘know’ when to initiate sleep and when to end it, and what can we do to help it along the way? To answer these questions, we need to delve into the world of circadian rhythms.
A circadian rhythm is simply a scientific term for a biological cycle that occurs once every 24 hours. There are also circannual rhythms which, as their name might suggest, occur once every 12 months. The sleep-wake cycle is not the only circadian rhythm – even plants and microbes follow circadian rhythms so as to adapt their biological functions to the night/day cycle. These rhythms usually respond mainly to the presence and absence of sunlight.
In humans, the sleep-wake cycle is controlled by a network of hormones, brain activity and environmental cues. These changes are orchestrated by a ‘master clock’ located in an area of the brain called the suprachiasmatic nucleus (SCN). Cells in this area maintain an approximate 24 hour rhythm through a rise and fall in activity of different ‘clock genes’, which can occur independently of any environmental cues, light or otherwise. This means that if you were to live in a cave entirely devoid of light, you would still follow a circadian rhythm, though studies suggest that this rhythm ‘drifts’ forward by about half an hour each day.
Cave-dwellers aside, this 24 hour rhythm is aligned (or entrained, to use the more popular term among scientists) to various environmental factors, with the most important one being light. You can think of this as the brain making adjustments to its internal clock in response to various external cues throughout the day. The brain has no inherent understanding of what time is – all it can do is use the information available to it to determine the current stage of the day/night cycle. The effects of this interaction are most obvious during jet lag, as it usually takes a few days for our master clocks to become fully entrained to a new light/dark cycle.
The term ‘Sleep-Wake cycle’ implies that this cycle is mainly an on/off switch for sleep, but this would be doing it a disservice, as sleep is far from the only thing that the sleep-wake cycle controls. Virtually all systems within the body are influenced by this circadian rhythm in some way. The prevailing stage of the sleep-wake cycle affects your alertness and perceived energy levels, your mood, your appetite, the control of blood sugar and cholesterol, the activity of the immune system and perhaps even the ability of your cells to repair damaged DNA.
This is important to be aware of, because it means that the optimal time of day to undertake certain activities such as exercise, eating or cognitively demanding tasks will depend on the timing of your circadian rhythm. Moreover, not everyone’s circadian rhythm follows the same pattern. For some people, alertness will peak much later during the waking period of the sleep-wake cycle. We’ll discuss how you can figure out what type of person you are, and how you should act on that information, in part 3. For now, let’s take a look at what the typical 24 hour cycle should look like.
There are two main environmental cues that trigger wakefulness in the morning: light and heat. Both exposure to light (particularly sunlight) and a slight rise in core body temperature triggers the release of cortisol, most commonly known as the stress hormone. Cortisol increases your metabolic rate, promotes alertness, and also puts in motion a timer that will affect when you start to feel sleepy later in the day. In addition to promoting cortisol release, seeing sunlight early in the day will also suppress the sleep-promoting hormone melatonin, adjust the phase of the master clock and, consequently, determine when the rhythmic release of melatonin will start to pick up again later in the day.
Notice how these environmental cues and their resulting hormonal changes don’t just serve to wake you up – they also serve as reference points for when sleep onset should occur. This means that the first step towards getting a good night’s sleep is actually to wake up correctly, which mainly means seeing as much sunlight as possible soon after waking up. We’ll discuss this in more detail in part 3.
While we are awake, our cells expend much more energy than when we are asleep. This energy is consumed in the form of the universal ‘cellular fuel’ called ATP, which is generated by breaking down various nutrients. After an ATP molecule is used for energy, it can either be recycled to create more ATP, or it can be further broken down to produce a molecule called adenosine. Adenosine is an important building block of DNA, but it’s also a sleep-promoting neurotransmitter. As energy is expended throughout the day, adenosine levels in the brain increase, resulting in increasing fatigue and ‘sleep pressure’. This is thought to be one of the reasons why exercise promotes sleep, as the increased energy expenditure results in more adenosine being produced. Caffeine keeps you awake and alert because it has a similar structure to adenosine, which allows it to occupy adenosine receptors and prevent adenosine from activating them.
As the angle of the sun in the sky decreases towards evening, the wavelengths of light present change. This alters the signals occurring in the brain and acts as another reference point for the circadian rhythm, indicating that the time for sleep is approaching. As the evening comes about and progresses, these changes result in the release of a sleep-promoting hormone that most people have heard of: melatonin. Melatonin release increases according to the prevailing phase of the sleep-wake cycle. Melatonin release is also very sensitive to the presence of light. Once natural light levels begin to decrease, exposure to even small amounts of light (and especially blue light) can interfere with melatonin release.
For humans, the presence of light is by far the most important factor for entraining the master clock, but there are a few factors that play a minor but noteworthy role. The timing of certain activities such as exercise, meals and even social activities can serve as reference points for the sleep-wake cycle. Evidence suggests that timing these activities in a consistent manner (eating lunch at the same time each day, for example) also leads to better sleep, while varying their timing can disrupt the sleep-wake cycle.
Prior to sleep, adenosine levels are at their peak, cortisol is low and melatonin is relatively high. The final trigger for sleep is a decrease in core body temperature. This coincides with an increase in circulating melatonin and helps the body enter the initial stages of sleep. This might seem counterintuitive given that warm baths, showers and hot water bottles are often helpful for relaxing and getting to sleep. This is because heating up the skin and the extremities causes peripheral blood vessels to dilate, diverting blood and heat away from core of the body. In other words, limited exposure to heat actually helps to decrease the core body temperature.
After sleep onset, adenosine steadily decreases throughout the night. Though not all research agrees, adenosine seems to suppress REM sleep, so this may be one of the reasons why REM sleep increases as sleep progresses. Melatonin peaks during the first half of sleep and helps to maintain sleep throughout the night. It decreases during the second half of sleep, while cortisol gradually increases from the onset of sleep towards its peak in the morning. The levels of these chemicals all contribute to the depth, quality and correct cycling of the sleep phases discussed during part 1. Thus, disruption to the levels of these chemicals (due to light exposure or increased body temperature, for example) will not only make it harder to get to sleep, but may also reduce the amount of time spent in the most important phases of sleep (REM sleep and phase 4 sleep).
In part 1, we covered what sets good quality sleep apart. Now we know the main factors that control when sleep begins and ends. In part 3, it will be time to put this information together and discuss in more detail the practices you can adopt in order to sleep as effectively as possible.
Daily rhythms of the sleep-wake cycle: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3375033/