Hair grows about 10% faster during the summer than in the winter.

The human hair growth cycle consists of three main phases: anagen (growth), catagen (transition), and telogen (resting). Research indicates that environmental factors like temperature and light exposure directly influence the duration and speed of the anagen phase. During summer months, higher ambient temperatures lead to vasodilation, where blood vessels expand to help the body cool down. This increased peripheral circulation significantly boosts the delivery of essential nutrients and oxygen to the dermal papilla at the base of the hair follicle. A study published in the British Journal of Dermatology noted that hair follicles are most active in the summer, with the highest percentage of hairs in the anagen phase occurring around July in the Northern Hemisphere. Conversely, the transition to the telogen phase peaks in late summer and autumn, leading to increased shedding. Sunlight exposure also increases the body's production of Vitamin D, which is vital for creating new hair follicles and maintaining existing ones. Hormonal changes, specifically fluctuations in prolactin and melatonin levels driven by longer daylight hours, further contribute to this seasonal growth spurt. While the average growth rate is 1.25 centimeters per month, the summer boost can increase this by roughly 10% to 15% depending on the individual's health and climate.