We each have an inner clock that influences when we feel like sleeping and waking and how tired we are. This clock in the brain tends to run slower than the 24-hour clock tied to the solar day — in fact, depending on genetics, it could be off by an hour or more. The inner, or circadian, clock controls the production of the hormone melatonin, which promotes sleepiness. When melatonin is delayed, you may suffer from insomnia. It also means the hormone may still be in your system when you want to start your day. This can cause fatigue and poor concentration, and leave you at risk for depression.
Take the 10 question quiz to see what kind of sleeper you are. (NYTIMES)