A reader asks:
Why do I always wake up at 4 a.m. no matter when I have gone to sleep?
You have a strong internal circadian rhythm signal for sleep offset! Two major independent factors influence the time we feel ready for sleep and when we awaken spontaneously (without alarm clock): the amount of accumulated sleep debt — which often increases during the work week — and an alerting signal from our biological clock. Although both factors are always at work, someone with a relatively strong internal clock signal will wake up at the same time whether sleep debt is high or low. The timing of the internal wake-up signal can be adjusted by light. Assuming that premature awakening at 4 a.m. is not due to depression or anxiety, bright light therapy in the evening shortly before habitual sleep time can set the clock to wake you up at a later hour.