People who take up smoking may do so because they are getting up too early. That's just one symptom of a permanently misaligned body clock, a condition dubbed "social jetlag" that could affect more than half of us.
Till Roenneberg at Ludwig Maximilians University in Munich and his colleagues used questionnaires to assess the "chronotype" of more than 500 volunteers - a measure of how much of a night owl or early bird you are. As with previous large studies, they found that the average person prefers to sleep between 12.30 am and 8.30 am, although chronotypes vary so widely that the latest owls are still awake when the earliest larks are rising.
Roenneberg says that for many people this natural rhythm, which is largely genetically determined, doesn't fit comfortably with their work routines. Although exposure to daylight can adjust people to even the most unsuitable routines for their chronotype, most office workers get nowhere near enough light to do this. Roenneberg believes that more than half the population are in effect socially jet-lagged all the time, because their body clock is permanently out of synch with their working hours.