Elizabeth Klerman is a sleep researcher and a professor of neurology.

It varies by person. Generally, it’s how much you get if you don’t have an alarm clock or someone to wake you up. See how much sleep you are getting after three or four days when no one wakes you and you don’t quickly get out of bed after you wake up (so you may fall back asleep). That might be how much you “need.” You can’t sleep unless you need to, not even when you’re bored — unlike eating chocolate cake, which you can do when you’re not hungry.

If you wake up and you’re not feeling rested, even after several nights of eight hours of sleep, you should consider seeing a sleep doctor. Not feeling rested could signal everything from narcolepsy to hypersomnia to sleep apnea. I’m not talking about how rested you feel the minute you wake up. Different parts of your brain wake up at different rates, so it’s not expected that you immediately feel totally alert. See a doctor if you’re waking up in the middle of the night or if your bed partner complains that you’re snoring loudly or that you’re kicking a lot.

Not everybody can get all the sleep they want at night. If you can take a nap, especially if you’re working the night shift, that’s good.

If you’re having problems falling asleep, don’t watch a horror movie before bed. No caffeine beforehand. Your body takes a while to figure out it’s time to go to bed. If you remember when you were a kid, you got a bath, you read a book, and the lights were low. The kid’s body says, “Now I know it’s time to go to sleep.” So when people are having problems going to sleep, we sometimes suggest doing something calming before getting into bed.

Melatonin is not regulated by the FDA. It’s a supplement, so you don’t know if what’s on the bottle is what you’re getting. I cannot suggest that people take melatonin unless they get pharmaceutical-grade. Other drugs, like more conventional sleeping pills, such as benzodiazepines, are not supposed to be taken long-term. They’re short-term solutions for a particular stressor.

For insomnia, long-term cognitive behavioral therapy is the way to go. As for sleep podcasts or sound machines, if that’s what works for people, I’m not going to object, especially if the sound turns off after a little while. Eye masks and earplugs, as long as they don’t block out something like a fire alarm, are fine with me.

— As told to Anna Lamb/Harvard Staff Writer