What is sleep?

Sleep is a state of human brain activity where the conscious connection to existence is broken. During sleep, the body recovers and rests, and for example heartbeat slows down and blood pressure drops. Only the brain works, as it processes the events of the day and recharges its own energy reserves.

Sleep stages are classified into dozing off period (N1 sleep), light sleep (N2 sleep), deep sleep (N3 sleep) and REM sleep.

A dozing person does not always feel like he is sleeping. In the N1 sleep, there are slow eye movements, and the brain’s 8-12 Hz alpha activity during wakefulness changes to 3-7 Hz theta activity. External stimuli, for example disturbing sounds, easily cause wakings. As a person who sleeps well, N1 sleep is less than 5 % of the night’s sleep. More than 10 % N1 sleep indicates poor sleeping quality.

In light N2 sleep, electrical activity of the brain slows further and there are observed 12-14 Hz sleep spindles, which are important for memory functions. N2 sleep makes up 35–55 % of the total duration of night sleep.

Deep N3 sleep (delta sleep) is the most important sleep. During deep sleep the energy reserves of the cells are filled, and the damage caused by the stress during waking hours is repaired.

In deep sleep, the brain’s energy reserves are also filled, and on the other hand, during wakefulness accumulated waste products are removed from the brain through the so-called cycle of the glymphatic system. The word “glymphatic” refers to the “lymph circulation” of the brain’s supporting cells, in other words glial cells.

According to the latest research, states of exhaustion in the brain, “brain fog” and also, for example, abnormal sleepiness causing Kleine-Levin Syndrome, could be caused by a disruption of the brain’s glymphatic circulation, whereby tiring waste products accumulate in the brain. Due to the long-term lack of deep N3 sleep, the glymphatic circulation can be disturbed, when amyloid also accumulates more easily in the brain. The accumulation of amyloid in the brain relates to i.a. memory disorders.

Deep sleep is also important for the conscious “hippocampal” memory. In deep sleep, a person “sleeps like a log”. Breathing is steady and pulse is calm. Deep sleep occurs most during the first third of the sleep phase (sleep time). For an adult who sleeps well, deep sleep is 15-30 % of the night’s sleep.

During REM sleep, brain and body are activated compared to slow-wave sleep: brain activity is intense, and breathing, heart rhythm and blood pressure vary. Even a healthy person can have breathing disruptions (apnea) during REM sleep. The day’s experiences and emotional states are organized and may repeat themselves. REM sleep is especially related to emotional memory and skill memory. It is also important for mental balance. Most eventful dreams are seen in REM sleep. During REM sleep, the brain metabolism is even more intense than when a person is awake. The eyes move quickly (Rapid Eye Movements) and the muscles are limp, but from time to time there occurs small muscle twitches. An adult who sleeps well has REM sleep for 20–25 % of the night’s sleep.

Antidepressants used to treat depression reduce REM sleep. Fatiguing antihistamines and very small doses of doxepin or mirtazapine, which block histamine-1 receptors, increase the amount of deep sleep without clearly affecting REM sleep. Benzodiazepines and many other sleeping pills can increase N1 and N2 sleep and reduce both deep N3 sleep and REM sleep.

The need for sleep and the sleep-wake rhythm

The amount of sleep a person needs varies individually. According to average values most people sleep 7-8 hours a day. In Finland, 4–12 % can survive on less than 6 hours of sleep (natural short sleepers), and 5–10 % need more than 9 hours of sleep per day (natural long sleepers) to be alert during the day.

Small children need a lot of sleep, and they also take naps. Teenagers should sleep even more than 9 hours a day, because at this stage of development, the second growth phase of the brain begins, and a lot of sleep is needed. In old age, the amount of night’s sleep may decrease, and nighttime awakenings may increase. On the other hand, in case of the elderly, napping partially compensates for shortened night sleep. A person has probably slept enough if he feels that the night’s sleep was refreshing when he wakes up in the morning.

Sleep is regulated by an internal circadian rhythm, which is synchronized by i.a. light, working life and various regular routines of the day. The rhythm can start to advance or delay individually, if it is not synchronized daily. In particular, morning light is an important synchronizer of the brain’s internal clock. The theme music of the evening news tells the brain that it is 8:30 pm. In the farmhouse, the rooster’s crowing tells you that it is early in the morning.

Sleep-wake rhythm disorders are quite common causes of insomnia. For example, a delayed sleep phase is common for young people, where sleep does not come until the morning, and waking up in the morning is difficult. On the other hand, the level of alertness is at its highest in the evening. In an advanced sleep phase disorder, you go to bed already between 19:00 and 21:00, and wake up in the early morning hours and can’t sleep anymore.

People are also naturally either morning or evening chronotypes, which is good to consider when planning your own circadian rhythm. The sleep rhythm can also get messed up due to irregular working hours and when traveling from one time zone to another.

Sufficiently long sleep and the good quality of sleep are essential for health. A person has been able to stay awake without stimulants under experimental conditions demonstrably for 11 days. In 1964 sleep researchers of the Stanford University confirmed that californian Randy Gardner was staying awake by monitoring the electrical activity of the brain. Staying awake longer has also been reported, but there is no solid evidence for them.

The ability to function after temporary staying awake 24 hours continuosly corresponds to a 1 per mille alcohol intoxication and staying awake 36 hours corresponds to a 1.5 per mille alcohol intoxication.

Long-term sleep deprivation (lack of sleep and/or long-term poor quality of sleep) can cause central obesity. The continuous increase of waist circumference can be a symptom of poor quality of sleep or lack of sleep at night. Long-term sleep deprivation also increases the risk of developing adult-onset diabetes, metabolic syndrome and hypertension. According to some research, it is also a risk factor for myocardial infarction and cerebral infarction.

Lack of sleep weakens immunity. According to research, untreated insomnia and sleep apnea have increased the risk of a coronavirus infection and prolonged long covid. A significant and long-lasting (lasting several years) lack of sleep can also be a risk factor for depression and memory disorders. Potential untreated sleep apnea together with insomnia further increases such impairments.

Source: Hyvä uni 2023 (Magazine Good sleep 2023)/professor Markku Partinen

How to help your sleep?

https://www.mielenterveystalo.fi/en/self-help/self-help-program-insomnia/2-tools