Is injury risk also highest at 05:00?

Not exactly. A 2006 analysis of industrial injury data put the daily peak in risk near midnight, earlier than the low in alertness. Sleepiness and injury risk are related, but they do not peak together.

Does a 12-hour night move the low?

No. A 19:00 to 07:00 shift crosses the same early-morning hours. In the model its lowest point is the same 5.0 at 05:45 if you got up at 07:00.

Does the low move for night owls?

In the model, yes. It moves the body clock 1 or 2 hours later for evening types and 1 or 2 hours earlier for morning types, a setting you can try in the shift alertness calculator.

The circadian low: why the early morning hours of a night shift are hardest

Updated October 8, 2026 · 2 min read

The early morning hours of a night shift, roughly 02:00 to 06:00, are the hardest because two pressures peak together: the body clock's alerting signal reaches its daily low near 05:00, and if you got up that morning you have been awake about 20 hours. FAA rules call 02:00 to 05:59 the window of circadian low.

Two clocks line up against you

Sleep pressure builds the longer you are awake. A night shift usually starts 10 to 16 hours after you got up, compared with 1 to 2 hours for a morning shift and 4 to 6 for an afternoon shift.

The body clock adds a daily wave on top. By day its wake signal rises and keeps you going. At night it falls, so both push you toward sleep.

The body-clock part of predicted alertness over 24 hours in the three-process model: lowest near 05:00, highest near 17:00, with 23:00 to 07:00 shaded.

NIOSH's training for nurses places the strongest sleepiness on night shift between 02:00 and 06:00, and the strongest drive for sleep between 02:00 and 05:00.

Worked example: a first night, 23:00 to 07:00

You get up at 07:00, skip a nap and start at 23:00. The three-process model predicts:

Predicted alertness through a first night shift (7 is the sleepiness line)
Time	Hours awake	Predicted alertness
23:00	16 h	8.7
01:00	18 h	7.0
03:00	20 h	5.8
05:00	22 h	5.1
05:45	22.75 h	5.0, the lowest
07:00	24 h	5.1

Predicted alertness for a first night shift, 23:00 to 07:00, after waking at 07:00 with no nap: below the sleepiness line (7) from about 01:15, lowest about 5.0 at 05:45.

Alertness keeps sliding for hours after it crosses the line and barely recovers by 07:00. The end of the shift, with handoff and the drive home, sits inside the low.

The same hours show up elsewhere

FAA flight crew rest rules define the window of circadian low as a period of maximum sleepiness between 02:00 and 05:59.
NIOSH reports that most drowsy-driving crashes and near misses happen between 04:00 and 06:00.
NIOSH also notes that night workers are at maximum risk of sleepiness at the end of the shift, which is the time of maximum risk for drowsy-driving crashes.

What helps in the low hours

A nap before the first night. NIOSH describes afternoon or evening naps of 1.5 to 3 hours that improved alertness on the night shift; see napping before a night shift.
A short nap on shift where allowed. In a randomized trial in an emergency department, a 40-minute nap opportunity at 03:00 left physicians and nurses with fewer lapses on a vigilance test at 07:30.
Light timing. NIOSH suggests bright light in the first half of the shift and less light in the second half, to make day sleep easier.
Fewer nights in a row. Injury risk rose with each of the first four nights in pooled industrial data; see consecutive night shifts.

Capillary's Analytics draws your hardest shifts over the 48 hours before them with predicted alertness, so you can see where your own lows fall.

Common questions

Is injury risk also highest at 05:00?: Not exactly. A 2006 analysis of industrial injury data put the daily peak in risk near midnight, earlier than the low in alertness. Sleepiness and injury risk are related, but they do not peak together.
Does a 12-hour night move the low?: No. A 19:00 to 07:00 shift crosses the same early-morning hours. In the model its lowest point is the same 5.0 at 05:45 if you got up at 07:00.
Does the low move for night owls?: In the model, yes. It moves the body clock 1 or 2 hours later for evening types and 1 or 2 hours earlier for morning types, a setting you can try in the shift alertness calculator.

See your own shifts this way.

Bring in your schedule from any scheduling app with its calendar link. Capillary charts your hours, nights, quick returns and hardest shifts. Only you can see it.

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