When it gets dark outside, melatonin begins to be produced: the hormone that makes us feel sleepy. Our natural biological clock tells us that it is soon time to sleep, so activities should wind down. When a person falls asleep, hormones start to produce. Sleep produces STH [growth Hormone](https://ul.orna.me/KOge/librarybiomarker?id=69), which helps children grow, and in adults is responsible for metabolism, affects mental health, and counteracts premature aging of the body. Low STH leads to obesity, and its levels are directly related to the levels of the ghrelin and leptin hormones. STH levels affect appetite, and a person with an imbalance eats more than he or she normally does. Levels of [cortisol](https://ul.orna.me/KOge/librarybiomarker?id=43), the stress hormone, normally diminish before bedtime and increase by the morning hours, awakening the body and stimulating activity. An increase in cortisol causes the pancreas to produce [insulin](https://ul.orna.me/KOge/librarybiomarker?id=67) to make the body want food in the morning. A disruption in this system leads to sleep disturbances (if cortisol hasn't gone down) and eating disorders (you've probably heard of people who can't eat breakfast but overeat at night). Sleep is also important for the immune system: lack of sleep reduces the production of the hormone prolactin, which directly affects the immune system and nervous system. ## How much sleep does it take for all the systems to work properly? - Newborns up to 2 months: 11–18 hours - Infants up to 1-year-old: 10–14 hours - Children up to 5 years old: 12–14 hours - Children up to 12 years old: 10–12 hours - Adolescents from 12-18 years: 8–10 hours - For adults (adjusted for individual differences, daily workload, physical activity): 7–9 hours ## Is it important to sleep during certain hours? The optimal time for adults to sleep is from 10:00 pm to 6:00 am so that you go to bed when it is dark and wake when it is beginning to get light out, but recent studies show that some adults may do better going to bed and waking up earlier or doing so later. It is worth noting that daytime sleep should not be substituted for nighttime sleep: if you nap for one hour, you should not cut your nighttime sleep by one hour. Having a regular sleep schedule is important for the proper functioning of hormones throughout both night and day.