Role of the brain in sleep

• In order for us to sleep the brain has to control various endogenous sleep-regulating substances the most important of which are melatonin adenosine and cortisol.
• Melatonin is known as the sleep hormone. It is controlled by the pineal gland and is released into our bodies in order to make us feel sleepy and eventually fall asleep.
• Melatonin is affected by sunlight. As the sunlight decreases at the end of the day the optic nerve in our eyes sends a signal to the SCN through a nerve pathway. The SCN sends signals to the pineal gland to start producing melatonin. Melatonin levels are at their highest in the middle of the night and start to reduce in the morning when sunlight appears.
• Another important chemical in the sleep-wake cycle is adenosine. Adenosine builds up in the brain during our waking hours which increases the pressure to sleep the more it accumulates. This means by the end of the day adenosine is at its highest and we will feel tired. This pressure to sleep is only relieved by sleeping. During sleep the adenosine decreases and is replaced with glycogen which gives us energy for the following day.
• Cortisol is commonly known as the stress hormone: it controls our fight-or-flight response if we identify a stressor. However another important role of cortisol is to help regulate the sleep-wake cycle. Cortisol levels rise and fall throughout a 24-hour period which helps us to fall asleep stay asleep and wake up. In the morning our cortisol levels greatly increase which gives us a boost of energy first thing which usually lasts between 30-45 minutes.