Here's a more detailed explanation:
* Continuous Growth: Hair grows in cycles, but within each cycle, growth is generally happening all the time, both day and night.
* Hormonal Influence: Some hormones that influence hair growth, like melatonin, are produced at higher levels during sleep. However, the effect is not so dramatic that it would mean hair *only* grows at night.
* Circadian Rhythm: Your body's natural sleep-wake cycle (circadian rhythm) does influence many bodily functions, including cell division and repair. It's possible that some aspects of hair growth are slightly more active during sleep, but this is a subtle difference.
* Perception: You might notice hair growth more at night or in the morning simply because you're not paying attention to it during the day. A small amount of growth accumulated over several hours can be more noticeable at the end of the day or after waking up.
In Summary: While there might be some subtle hormonal or circadian influences that could potentially boost hair growth slightly during sleep, hair growth is an ongoing process that happens both day and night. It doesn't exclusively occur at night.