The afternoon nap: twenty minutes to recharge.

An afternoon nap does you good when it's short and early: twenty minutes, alarm set, six to eight hours after you get up. It can restore focus, memory and mood without touching your night. Let it run to an hour or slip into late afternoon and you're likely to wake groggy, then struggle to drop off at bedtime.

Quarter past one. Lunch with the client is over, the board call is at half two, and the car looks like a bed. I see it a lot in the people I work with: they either push on with coffee or sleep for an hour in the driver's seat and come back feeling worse.

Is an afternoon nap good for you?

Yes, if it stays short. A power nap, a brief sleep in the early afternoon, helps you think clearly again. On Ikaria, the Greek island known for long lives, it's a daily habit. In a monotonous driving test, people who'd napped for under fifteen minutes made 9% fewer mistakes. It goes wrong in three cases.

• When it runs long. Past half an hour you sink into deep sleep and wake up heavy and muddled. That's sleep inertia.
• When it comes late. Your need for sleep builds through the day. A late-afternoon nap uses some of it up, so you take longer to drop off at night.
• When it stands in for the night. It rests you, but the hormonal work of night-time sleep still belongs to the night.

How long should an afternoon nap be?

Twenty minutes, which usually keeps you in light sleep. The range runs from ten to thirty; beyond that, you're more likely to wake up groggy. If you only have five or ten, take them: they already clear your head a little. Ninety, a full cycle, only after a really short night.

Set the alarm before you close your eyes. Twenty minutes, or twenty-five if it usually takes you about five to drop off.

The time, the place and the wake-up

The time: six to eight hours after your alarm. That's the natural dip, when digestion asks your body to slow down. Up at half six? Between half twelve and half two. If you've missed the window, skip it.

The place: dark, quiet, phone silenced. At the office, door shut and lights off; in the car, seat back and parked in the shade. Keep an eye mask and earplugs in your work bag, breathe slowly and let your shoulders drop.

Coffee just before. Have it straight after lunch, then close your eyes. Caffeine takes thirty to forty-five minutes to kick in, so it arrives with the alarm. In the same test, people who had caffeine before their nap made about a third fewer mistakes.

If sleep doesn't come, it still counts. Ten to twenty minutes with your eyes closed, slow breathing and no screens is already deep rest. If you struggle to fall asleep at night, it's the right choice: rest now, and save the tiredness for bedtime.

Done this way, a nap gives you a second start to the day. The hours that truly recharge you are still the ones at night.