It can be a long process — much longer than the 21 days that's often promised. Norman and Tegan tell us how to form a new habit, in far less than 300 pages, so you can get started.

Habit formation is the process by which behaviors become automatic. Habits can form without a person intending ... running shoes after getting home each day or buckling your seatbelt without ...

For some, it took as little as 18 days. For others, it took as long as 254 days to form a new habit. Greco said there are a variety of reasons why it takes some people longer to form a new habit.