A reader asks:

How can I determine the optimal number of hours of sleep for me? I have noticed that when I wake up 30 minutes earlier than my usual time, I feel more refreshed than on other days. However, I am not sure whether that is due to the amount of sleep I am getting–less than usual–or the time I am waking up.


Every person has an individual sleep need, but we have no direct way to measure it. Many people sleep longer than necessary or desirable, often on the erroneous assumption that more sleep leads to greater daytime energy and alertness. For people vulnerable to depression, excess sleep can be depressogenic–that is, it can trigger blue mood and increased daytime fatigue. In such cases, shorter sleep is actually advantageous. If you find that waking 30 minutes earlier than usual makes you feel better, that is a strong clue that you are sleeping too much. You can test this by waking earlier for a whole week, to ascertain whether the accumulated sleep reduction begins to make you sleepier, rather than more energetic.

Many people get inadequate sleep during the week, because of the pressure of work schedules. They find themselves needing “recovery sleep” on the weekends. For those people, additional reduction of weekday sleep time is not recommended. To further answer your question, the critical factor is probably the amount of sleep you are getting, not the time you are waking up. On the other hand, waking up to bright morning light, whether from outdoors or timed artificial light sources, is far more healthy than waking in a dark bedroom.