Sleeping well

When we sleep, the brain continues to be highly active and takes the opportunity to carry out “brain maintenance” tasks, such as:

• Repairing and organising neural connections.
• Renewing proteins and receptors.
• Removing waste products that have built up during the day.

This “brain maintenance and repair workshop” is essential for memory, learning and brain health.

Some neural systems that promote wakefulness gradually switch off, while others that induce sleep are activated in sequence. For one system to take over, the other must stop being active.

During the state of alertness, the brain (particularly the cerebral cortex) remains active thanks to various neurotransmitters (chemical substances) such as hypocretin, noradrenaline, adrenaline, serotonin, histamine and acetylcholine. These substances play a key role in maintaining wakefulness.

As the need for sleep increases, substances such as adenosine, the melanin-concentrating hormone, GABA and glycine build up. These substances help to inhibit the systems that keep us awake and facilitate the transition into sleep.

The circadian rhythm is the body’s internal biological clock. It regulates functions such as sleep and wakefulness, body temperature, hormone production and metabolism over a cycle of approximately 24 hours. This rhythm is influenced mainly by the light and darkness in our environment. This biological clock is unique to each person and is made up of cells in the brain that communicate through electrical signals.

Every 24 hours, this biological clock sends a signal to the pineal gland to start producing melatonin, a hormone that plays a key role in regulating sleep. Melatonin levels increase as it gets dark, promoting sleepiness, and decrease with the morning light, making it easier to wake up. This process is controlled by a structure in the brain called the suprachiasmatic nucleus.

Melatonin is synthesised from tryptophan, an amino acid that we obtain through food. Tryptophan is converted into serotonin, which acts as a precursor to melatonin and also plays a role in regulating wakefulness and the onset of sleep. When melatonin is synthesised, it is released into the bloodstream and is rapidly metabolised, mainly in the liver, before being eliminated in the urine.

Cortisol, on the other hand, follows a circadian rhythm that is the opposite of that of melatonin. Cortisol levels remain low during the night and rise towards morning, helping us to wake up and become more alert.

Hypocretin (orexin) is produced in the hypothalamus. Its role is to keep us awake over a sustained period; in other words, it acts as a “switch”. When there is a deficiency of this hormone, a person may fall asleep unexpectedly and enter the REM phase directly. This condition is known as narcolepsy.

Other factors, substances or hormones that regulate sleep:

• Homeostatic factors. These are the mechanisms that regulate our need for sleep depending on how long we have been awake. The longer we stay awake, the greater our sleep pressure becomes, partly because of the accumulation of adenosine.
• Neurotransmitters. These are chemicals in the brain that help regulate the balance between wakefulness and sleep. Some promote wakefulness (such as hypocretin or noradrenaline), while others promote sleep (such as GABA).
• Hormones and energy metabolism. These are molecules and hormones that link sleep with hunger (the body’s energy balance). Ghrelin levels increase when we are sleep-deprived, stimulating appetite, while leptin levels decrease, reducing feelings of fullness. This mechanism helps to explain the link between lack of sleep and conditions such as obesity.
• Magnesium. Although it is not a hormone, it plays a role in regulating sleep by helping to promote relaxation of the nervous system and muscles.