The mid-afternoon slump isn't really about lunch. It shows up even when people skip the meal (Monk, 2005). Two things overlap: sleep pressure, which has been building since you woke up as adenosine accumulates in the brain, and a small dip in the body clock's alerting signal in the early afternoon. For a while the clock's push toward wakefulness can't quite keep up, and you feel it as heavy eyelids and slow thinking. Read more in why you crash at 2pm.
Because the dip follows your own day, the most useful input is when you woke up. This calculator places it about six and a half to eight and a half hours after waking. That's a typical estimate; your own dip can land earlier or later.
Power Nap, Full Cycle or Caffeine Nap?
Power nap (10–20 minutes). The safest choice for most afternoons. You stay in light sleep, so there's little grogginess afterwards. In one study of sleep-restricted adults, a 10-minute nap gave the best balance of quick recovery and little grogginess (Brooks and Lack, 2006).
Full cycle (about 90 minutes). Useful after a short night, if you have the time. A full cycle ends in lighter sleep, so waking is easier than at the 45-minute mark. Start it early in the dip so it finishes well before the evening.
Caffeine nap. Drink a coffee, then lie down for about 20 minutes; the caffeine arrives as you wake. It only works if that coffee can clear by bedtime, so the calculator checks it against the same under-50 mg line the ARC app uses. Use the caffeine cutoff calculator to see your own last safe coffee.
When to Skip the Nap
If you struggle to fall asleep at night, naps can make it harder by releasing sleep pressure you need later. A short walk outside in daylight is a good alternative: it lifts alertness without touching tonight's sleep. And if you feel very sleepy during the day even after a full night, that's worth raising with a doctor.