Your hair grows fastest while you are asleep.

Hair growth is deeply tied to the body's circadian rhythm and the endocrine system. During the deep stages of sleep, specifically Stage 3 non-rapid eye movement (NREM) sleep, the pituitary gland releases a surge of human growth hormone (hGH). This hormone is vital for repairing tissues and stimulating the proliferation of matrix cells within the hair bulb.The hair follicle is one of the most metabolically active organs in the human body. Research published in the journal 'Nature' has shown that the biological clock within hair follicle cells regulates the timing of the hair cycle. When sleep is consistent, these cells can efficiently synthesize keratin, which is the structural protein that makes up approximately 65% to 95% of the hair shaft by weight.Chronic sleep deprivation leads to increased levels of cortisol, often called the stress hormone. High cortisol levels can prematurely push hair follicles from the anagen (growth) phase into the telogen (resting) phase, a condition known as telogen effluvium. This shift can result in noticeable hair thinning or shedding several months after the period of poor sleep.Furthermore, sleep promotes increased blood flow to the scalp, delivering essential nutrients and oxygen to the follicles. A study by the University of California, Los Angeles (UCLA) suggests that melatonin, a hormone produced during darkness, also plays a direct role in hair health. Melatonin has antioxidant properties that protect follicles from oxidative stress, ensuring the growth cycle remains uninterrupted.