The way a person's hair grows is unique to that individual, but there are certain factors that can affect the rate it grows.
Activity
Hair growth is connected to metabolism, so the rate a person's hair grows has more to do with his activity level. For people who keep a typical schedule, hair will grow slowest at night during sleep, when resting metabolism is at work.
Winter
Our metabolisms slow down in the winter to allow us to eat less during the cold months ahead. When metabolism is slowed, so is hair growth.
Follicles
Each follicle on a person's head operates on its own. If all of our hair follicles followed the same pattern, they all would fall out at the same time. Instead, it falls out, grays and grows at different rates.
Misconceptions
People sometimes think hair grows more slowly as it gets longer, but this is an illusion. Normal growth is simply more noticeable when hair is shorter.
Split Ends
Split ends can make hair grow more slowly. The only way to treat split ends is to cut them off with scissors before they travel farther up the hair.