How many hours should we sleep?

The differences between morning people (“larks”) and evening people (“owls”) are explained by our chronotype, that is, each person’s biological tendency to be more active at certain times of the day.

The chronotype depends on our circadian rhythm, the body’s internal biological clock regulated by the suprachiasmatic nucleus in the hypothalamus. It is also influenced by genetic, hormonal (particularly melatonin) and environmental factors.

Melatonin, which is released when it gets dark, helps induce sleep:

• Morning people produce melatonin earlier, making it easier for them to fall asleep sooner and wake up feeling energetic in the morning.
• Evening people produce melatonin later, meaning they tend to be more active and perform better in the afternoon or evening.

It is not simply a matter of habits or willpower, but rather a biological trait. This pattern can change with age and, in extreme cases, may be linked to genetic variations and considered a disorder.

As well as differences in chronotype, people also differ in the amount of sleep needed:

• Long sleepers usually need 9 hours or more of sleep to feel rested.
• Short sleepers can sleep for 6 hours or less without experiencing daytime sleepiness or cognitive impairment. 

Most people need between 7 and 8 hours of sleep.

Thus, the chronotype (lark or owl) determines when we are naturally more inclined to sleep, while whether we are a long sleeper or a short sleeper determines how many hours of sleep we need.