Human sleep is made up of cycles, and the cycles of phases – deep and shallow in turn. Although the cycles are of similar length, each has a different structure. Welcome to the architecture of sleep.
Human sleep happens in cycles. These are more or less similar in length: around one and a half hours. There are normally five cycles, each of which is important. Each sleep cycle has several phases. While we are still awake, muscle tension remains high. The brain’s electrical activity is fast and low voltage; different signals coming from different parts of the brain, which gives the impression of chaos.
From being awake, we pass into the first stage of NREM (non-Rapid Eye Movement) sleep. This is a light sleep during which tension in the body decreases, although sometimes there can be sudden twitches. Eyeball movement is slow.
In the second phase of NREM sleep, the eyeballs stop moving. Body temperature falls and the heart rate slows. There are sudden spikes in the electrical activity of the brain, so-called sleep spindles.
The third and fourth phases of NREM bring a deep, slow wave sleep (SWS). The eyeballs don’t move and muscle tension is low. The electrical activity of the brain becomes slower, synchronized and high voltage; different parts of the brain are attuned to each other.
Right at the end of the cycle, we enter the REM phase. This is the time when we dream. An external observer will clearly see the rapid movements of the sleeping person’s eyeballs. The rest of the body, however, remains completely relaxed. If this wasn’t so, we could hurt ourselves performing the movements we are dreaming about. Electrical brain activity is similar to levels we have when awake – it is unsynchronized and fast, creating the impression of chaos. The pre-frontal cortex, the part of the brain responsible for logical thought and behaviour control, is less active. However, the remaining regions of the brain are active.
Each successive sleep cycle is slightly different from the previous one. The first cycle is dominated by deep NREM sleep. Subsequent cycles have increasingly long, light NREM and REM phases. If someone goes to sleep very late at night, their sleep doesn’t start from the first cycle, but jumps straight to the next ones. As a result, they miss out on a large part of the important, deep NREM phase. It is, above all, during NREM sleep that our short-term memory is tidied up; memories are transferred to the long-term memory. Our motor skills are also consolidated. Insomnia is the inability to enter NREM sleep.
REM sleep helps us deal with trauma and disturbing memories. It also helps us interpret the impressions the brain receives while we are awake – for example, the correct reading of emotions from facial expressions. This is creativity training for the brain. This evolutionary invention is younger than short wave sleep (SWS). Apart from birds and mammals, we don’t know of many other animals that have this sleep phase.
Each person’s sleep pattern is slightly different. Around 10% of us are larks – people who wake early in the morning. 20% are night owls – people who fall asleep late at night. The rest of us are somewhere in between. Research shows that people who wake early are healthier and feel better. They rarely suffer from depression and show lower stress levels. This may be related to the fact that they have time to prepare better for the challenges of the coming day and devote more of that time to physical activity. Night owls are statistically more sociable, creative and open. However, they should remember to take exercise in order to reduce the negative health consequences of their lifestyle.
The daily cycle of humans is a result of the overlap of three different cycles. First, during the day, the concentration of melatonin in the blood stream (produced under the influence of darkness) fluctuates. For the average adult, the concentration of melatonin begins to increase significantly from about 10pm and falls in the morning. Second, the concentration of adenosine changes. This is a compound that builds up while we are awake and thanks to which our body knows how much sleep it needs. Receptors that measure the level of adenosine can be blocked for a while with caffeine, but adenosine will still accumulate and, when the caffeine stops working, drowsiness will return with increased force. Third, we have a rhythm imposed on us by society, which, as one can easily see, is not particularly consistent. We have to get up early for work and school, but social and cultural activities demand energy in the evenings.
In teenagers, melatonin starts to be produced an hour or two later than in most adults, sometimes making it hard for them to fall asleep before midnight. As a result, we hear scientists recommending that school should start later for this age group, in particular because good quality sleep is crucial for absorbing information. However, it doesn’t look as though anything is going to change here. So, for now, we should advise the young to switch off their smartphones and computers two hours before going to sleep; screens emit blue light, which blocks the production of melatonin.
Unlike teenagers, in older people melatonin is produced earlier than normal. A common problem for older people is that they fall asleep in the early evening, which reduces the concentration of adenosine. This then leads to a sleepless night. One can get round this by going outside in the late afternoon; the sun’s rays delay the production of melatonin.
Sources: Matthew Walker, “Why We Sleep”; Fiona Kerr, “Health Check: What Determines Whether We’re Night owls or Morning Larks?”
Translated from the Polish by Annie Jaroszewicz