The pattern and distribution (rhythm) of sleep during the day is formed by a person's biological clock. One of the most important factors that guide a person's biological clock is light. The light perceived by the eyes reaches the relevant center in the brain, which, depending on the amount of light (day-night), sends stimulating or inhibiting messages to the centers that secrete substances and hormones that cause sleepiness or wakefulness (in other words, things that disturb sleep). This means there are periods during the day when people tend to sleep or cannot sleep easily if they want to.
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