Sleep is regulated through the endocrine system (hormones) and the neurotransmiter adenosine.
Melatonin
Melatonin is often referred to as the sleep hormone as it has a central role in the sleep-wake cycle. It is synthesised and secreted from the pineal gland from a pathway that also includes tryptophan and serotonin. Melatonin is secreted at night, about 2 hours before a persons bedtime, signalling sleepiness and an urge to sleep, it reaches maximum levels around 3-4am and starts to decrease in the blood plasma signalling time to wake up.
Melatonin is regulated by the suprachiasmatic nucleus (SCN) and is therefore connected to the circadian rhythm.
Cortisol
Cortisol is known as the stress hormone as it’s produced in response to stress as part of the sympathetic nervous system by the hypothalamic pituitary adrenal axis (HPA axis), part of both the central nervous system and the endocrine system. It’s made up of the adrenal glands at the top of your kidneys and the hypothalamus and pituitary gland in your brain.
Cortisol keeps us alert and helps wake us up in the morning when cortisol is naturally at its highest and falls over the course of the day. Excessive stress can elevate cortisol levels late in the day causing difficulty sleeping.
Adenosine
Sleep drive is dependant upon the accumulation of the neurotransmitter adenosine. When we wake up, adenosine is low and it builds up throughout the day. The longer we are awake for the more adenosine builds up.
Adenosine is a by product from the breakdown of ATP our primary fuel source, so the more energy we need, for example through exercise, the more adenosine we create and consequently the more sleepy we become.