How to calculate sleep debt, and pay it back gently

Step 1: find your sleep need

Most adults need 7 to 9 hours of sleep a night. Your own need is fairly fixed, and it's the amount you'd settle into after a week or two with no alarm and no catching up to do.

A practical way to estimate it: on a holiday with no alarm, ignore the first few nights (you'll be repaying debt) and average what you sleep after that. Or start from 8 hours and adjust: if you're still sleepy in the afternoon after a fortnight of 8 hours, your need is probably higher.

Step 2: add up the gap

For each of the last 14 nights, subtract what you slept from your need. Add up the shortfalls. Nights where you slept more than your need pay a little back.

Recent nights affect how you feel today more than nights two weeks ago, so a good calculation weights them more heavily. Energy Curve does this: last night counts the most, a night two weeks ago the least.

Step 3: pay it back gently

• Go to bed earlier by 30 to 60 minutes for a week, rather than sleeping in.
• Keep your wake time steady, even at weekends, within about an hour. A big lie-in shifts your body clock and makes Sunday night harder.
• Nap early and short: 20 to 30 minutes before mid-afternoon. Long or late naps can make it harder to sleep at night.
• Get daylight in the morning, which helps keep your body clock on time.
• Stop caffeine about 8 hours before bed. It lingers longer than most people think.

Short-term debt can be repaid over a few nights. Months of short sleep aren't simply "paid off" at the weekend; the fix is a better routine.

Why your energy goes up and down

Sleep scientists describe alertness with a two-process model. Sleep pressure builds from the moment you wake and is cleared by sleep. Your circadian rhythm, the body clock, pushes alertness up and down over the day. Together they give most people:

• Grogginess for the first hour or so after waking (sleep inertia).
• A late-morning peak: good for focused, demanding work.
• An early-afternoon dip, deeper when you're carrying sleep debt.
• A second peak in the early evening, then a fall towards bedtime.

Timing hard work for the peaks and easy tasks or a walk for the dip makes the same amount of sleep go further.

Tracking it in Energy Curve

Energy Curve is a free sleep debt and energy tracker in your browser.

• Log sleep with "I just woke up" or "Going to bed" on the Energy tab, or add nights on the Sleep tab.
• Set your sleep need with the slider (6 to 10 hours), or use Find my need.
• Read your debt on the gauge and the last 14 nights chart.
• See today's curve: your peaks, the dip and the evening wind-down, with suggested times for caffeine, light, exercise and meals.
• Plan a bedtime from the time you need to wake up, or plan a nap.

Your data stays in your browser, with backup and CSV export. Energy Curve Pro, a one-time $2.99 upgrade, adds a jet-lag planner, a chronotype quiz and sleep sounds.

Common mistakes

• Counting time in bed as sleep. Subtract the time it takes to fall asleep.
• Sleeping until noon on Saturday. It shifts your body clock. Go to bed earlier instead.
• Napping late. A nap after about 3pm can push bedtime back.
• Using caffeine to cover debt. It hides the tiredness but doesn't repay it.
• Assuming you need less than everyone else. Very few people do well on under 6 hours.
• Ignoring persistent tiredness. If more sleep doesn't help, see a doctor.