An afternoon nap is one of the joys of life, although excessive napping could signal all is not well. A new study, published in the journal Alzheimer’s & Dementia, suggests that napping during the day could be an early sign of Alzheimer’s. Researchers compared the postmortem brains of 13 people with Alzheimer’s to the brains of seven control subjects and found those with the brain disease had a build up of a protein, tau, in areas of the brain involved with wakefulness.

“Some people nap all their life, no problem,” Lea Grinberg, professor of neuropathology at the University of California San Francisco, told the Times. “In some cultures, people will have a daily siesta – this is fine. The warning comes when people start sleeping during the daytime, when they did not before.”

It is not completely clear whether naps are good for you or not. Some studies suggest they reduce stress and increase cognitive function, but one study suggested naps were linked to an increased risk of mortality, and there is certainly good reason to believe that daytime sleepiness – as in the Alzheimer’s study – can be a marker of an underlying condition.

For many people, napping during the day is mainly a sign that you are not getting enough sleep at night, says Dr Neil Stanley, a sleep expert and the author of How To Sleep Well. “If you feel sleepy during the day, you should think about taking a nap. That is what the body needs – it doesn’t need to be kept awake with caffeine, it needs sleep.” The feeling to notice is “sleepiness”, he says, not “tiredness”, which could be more psychological and linked to stress.

So how do you nap well? The key thing, says Stanley, is duration. Choose either a 20- or 90-minute nap. “When you fall asleep, you’ll quickly go through the lighter stages of sleep into your first period of deep sleep. You don’t want to wake up in deep sleep because that’s when you wake and feel worse than you did before.” Napping for 20 minutes means you will wake up before you go into deep sleep; napping for 90 minutes means you’ll complete a sleep cycle.

Once you factor in the time it takes to fall asleep – some people are better at napping than others but, says Stanley, “a healthy adult will fall asleep in between five and 12 minutes” – you can set an alarm, allowing a 30- to 40-minute period for a short nap, and up to two hours for a longer one.

A good time to nap is during the body’s natural dip in the afternoon, between about 2pm and 4pm (older people’s circadian rhythms change, says Stanley, so they may start feeling sleepier earlier in the day). “You don’t really want to be napping much past that because then you are going to be eating into your night-time sleep,” he says. The point, he stresses, is to get good night-time sleep, which would ensure you probably don’t need to nap at all.