Blog · November 1, 2026

Energy through the day: how Aurek draws your curve

Rest & Refuel shows a curve of how alert you’re likely to feel today, with a few windows marked on it. Here’s exactly how it’s drawn, and what it can’t tell you.

General information, not medical advice. Check with a doctor before starting a new program, especially with an injury or a medical condition.

Two processes, one curve

Sleep scientists have long described alertness with two processes [1, 2]. Sleep pressure builds the longer you’re awake and clears while you sleep. Your body clock raises and lowers alertness on a roughly 24-hour rhythm, whatever you slept. The three-process model of alertness adds a third: sleep inertia, the grogginess right after waking [3, 4]. Aurek uses that model’s published numbers.

What goes in

• Last night and the three nights before, as real time asleep (a watch through Apple Health, a night you logged, or the phone’s estimate once you confirm it). A night with nothing logged counts as your usual times.
• Your usual wake time, from your recent nights. It sets the body clock, because one late morning doesn’t move it [5].
• Sleep debt from the last two weeks. The model alone forgets a short night within a day or two, but lost sleep adds up [6], so debt lowers the whole day a little (at most a set amount).
• Today’s naps, which count as sleep.

The windows

• Groggy: from waking until sleep inertia has mostly faded, about an hour [4].
• Peak focus: the top of the curve before the afternoon.
• Afternoon dip: a smaller 12-hour rhythm puts a low point in the early afternoon [7, 8]. Aurek only marks it when the curve really dips.
• Best time to train: strength and power tend to peak in the late afternoon and early evening, with body temperature [9]. The window ends at least two hours before bed [10]. If a workout is planned, it’s marked on the curve, with a word on whether it falls in a good window.
• Wind down: from your wind-down time to tonight’s bedtime, from the same sleep plan the app already uses.

What it can’t do

• It’s a model of a typical person with your sleep and your times. It doesn’t measure you, and it isn’t medical advice.
• Caffeine, illness, stress and a hard workout all change how you feel; the curve doesn’t know about them.
• With no sleep logged, it uses your usual times and says so.

Where it runs

The curve is worked out on your phone each time you open Rest. It isn’t saved, synced or sent anywhere, like the rest of your sleep data.

Sources

1. Borbély AA. A two process model of sleep regulation. Human Neurobiology. 1982;1(3):195–204.
2. Borbély AA, Daan S, Wirz-Justice A, Deboer T. The two-process model of sleep regulation: a reappraisal. Journal of Sleep Research. 2016;25(2):131–143.
3. Åkerstedt T, Folkard S. The three-process model of alertness and its extension to performance, sleep latency, and sleep length. Chronobiology International. 1997;14(2):115–123.
4. Tassi P, Muzet A. Sleep inertia. Sleep Medicine Reviews. 2000;4(4):341–353.
5. Roenneberg T, Wirz-Justice A, Merrow M. Life between clocks: daily temporal patterns of human chronotypes. Journal of Biological Rhythms. 2003;18(1):80–90.
6. Van Dongen HPA, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness. Sleep. 2003;26(2):117–126.
7. Monk TH. The post-lunch dip in performance. Clinics in Sports Medicine. 2005;24(2):e15–e23.
8. Lavie P. Ultrashort sleep-waking schedule. III. Gates and forbidden zones for sleep. Electroencephalography and Clinical Neurophysiology. 1986;63(5):414–425.
9. Drust B, Waterhouse J, Atkinson G, Edwards B, Reilly T. Circadian rhythms in sports performance: an update. Chronobiology International. 2005;22(1):21–44.
10. Stutz J, Eiholzer R, Spengler CM. Effects of evening exercise on sleep in healthy participants: a systematic review and meta-analysis. Sports Medicine. 2019;49(2):269–287.

Written by the Aurek team. This post describes how the app works today; the app’s own “Why?” sheets show the same numbers.