Because Ramadan is a lunar month, it begins about eleven days earlier each year. Ramadan thus serves as a positive physical challenge each year. Throughout a Muslim's lifetime, Ramadan will fall both during winter months, when the days are short, and summer months, when the days are long and the fast is more difficult. Accordingly, it is a test of ultimate devotion, in which physical endurance mirrors the inner spiritual state, which a believer constantly strives to improve.