The sleep records of 55,000 subjects establish an association between the body's sleep-wake cycle and the time of sunrise during standard time, but not when daylight-saving time is in effect. The observation suggests that dawn entrains our inner clock during standard time, but not during the summer months. The cause could be DST's artificial resetting of the clock. But it could also be that our internal clocks are tied to the coming of dawn only when the days are long, irrespective of DST. That's why the authors did the additional study of 50 subjects, which focused precisely on the change in the body's clock at the moment of DST transition. The change in sleep midpoint precisely at the spring and fall transition points is very striking. It adds credence to the idea that turning the clock backward or forward is somehow intimately associated with changes in the way our internal clock is set.