During sleep, our body switches into a kind of "standby mode" for recovery. But there's one part of our body that never sleeps: our brain. It's still active at night, evaluating what's gone on during the past day. Important events and information are sorted into existing categories, whilst unimportant things are deleted. Our brain cannot carry out this task when we are awake as otherwise we would not be able to record the information that we are taking in during the day – the processing of these stimuli would become confused, which could lead to hallucinations. Content memorised shortly before falling asleep is processed particularly well. This is especially beneficial to pupils and students when it comes to learning.
Our brain is largely responsible for preparing the body for the transition between day and night and for ensuring that we are awake and ready during the day and tired at night. Nerve cells that establish the connection to the optic nerve and other cranial nerves play an important role in this. The signals that are activated due to the change between light and dark or day and night impact the immune system and our hormones. At night, for example, the hormone melatonin, a hormone that is only secreted in the dark, is released and makes us feel tired. Growth hormones are also released at night, helping children to grow during sleep. These hormones are also responsible for better healing of wounds. This is why damaged tissue repairs itself more quickly at night. During the morning, as daylight increases, the secretion of melatonin decreases and it's replaced by cortisol, a hormone that makes us more awake. Our body temperature, which falls at night, increases again in the early hours. Blood pressure, heart rate and breathing likewise rise again in the morning, kickstarting our bodies into action for the day.
The hormone leptin which is also released during sleep enables us to go without food for eight hours or longer. As soon as we're awake again, the hormone ghrelin takes over which signals hunger to the brain. In the case of chronic sleep disorders, the balance between these two opposing hormones is often disrupted, which can lead to
people becoming overweight. People who are paying attention to their weight should therefore focus on having a balanced sleep.
During sleep, our immune system runs at full speed. Our defences are increased as more immune-active substances are secreted during sleep. This makes it easier to fight infections. As soon as we feel a flu we become tired and need a lot of sleep. The natural "killer substances" that are also activated during sleep have a positive effect on our immune activity. Our body tells us that we're tired so that our immune system builds up again during sleep and thus the recovery process accelerates during the course of the illness. People who don't sleep enough are at greater risk of having a weakened immune system and thus being more susceptible to becoming sick. A lack of sleep that stretches over a longer period can also increase complaints such as high blood pressure or gastrointestinal problems.
Our sleep also regulates the metabolism of all products that we have consumed during the day. Those who sleep too little run the risk of their metabolism not being carried out completely. This can lead to type II diabetes or obesity. Not enough sleep can inhibit the release of insulin, which can lead to a resistance to insulin. Its hormonal opponent, glucagon, which can raise blood sugar levels again, is secreted less. According to one study, the risk of type II diabetes increases for a sleep duration of less than five hours as well as for more than nine hours per night over a long period of time.