You were fine at 6am. By mid-afternoon you cannot keep your eyes open, and the drive home is worse than the shift was. That is not poor sleep hygiene and it is not lunch. It is a second alertness low that sits about eight hours after the hour you would wake with no alarm, and an early start is the roster that puts you awake and driving through it.
WHAT THIS PAGE WILL NOT TELL YOU
This page explains how the timing works and why. It does not print your six times, because they are not printable: they move with your finish time, your commute and the hour you wake with no alarm. Four questions, about forty seconds, no account.
- Your crash window
- Bright light: start and stop
- Sunglasses on: start and stop
- Main sleep: in bed and up
- Pre-shift nap: and when to set the alarm
- Last caffeine
FREE TO SEE YOUR CRASH WINDOW · NO ACCOUNT · 40 SECONDS
The slump is real, and it is not the sandwich
The afternoon dip has been measured in people who ate nothing at all, which is the finding that kills the lunch explanation. Broughton described it as a nap zone under circadian control, and Monk reviewed the performance data under the older name, the post-lunch dip. The size of it is modest in a well-rested person. In someone carrying sleep debt it is not modest, and an early start is a sleep-debt machine: shifts starting before 04:30 average about five hours of sleep a night. The dip and the debt land on the same afternoon.
Everything below is the mechanism. The six clock times it produces depend on your finish time, your commute and how early you wake unaided, so this page cannot print them. Get yours in about forty seconds.
Why yours lands at a different time from your colleague's
The dip tracks your temperature minimum, which tracks the hour you would wake with no alarm. Two people on the same early roster whose natural wake times differ by two hours have dips two hours apart, so one of them is at their worst while the other is still fine. The one whose finish time sits closest to their own dip is the one who should not be driving. Which of those you are is the first thing worth knowing and the only thing on this page that a clock on the wall cannot tell you.
The drive home
Across 135 early-start rosters this tool models, spanning 3am to 7am starts, eight to twelve hour shifts and three chronotypes, the drive home falls inside the afternoon dip on 72 of them. That is 53 percent, so more often than not. When it happens, the sequence that helps runs before the dip rather than during it: caffeine taken roughly 30 minutes ahead of the window works, caffeine taken inside it arrives late. The conflict is that caffeine has a half-life near five hours and your bedtime is already the hardest part of the day, so a dose that saves the drive can cost the sleep that would have prevented tomorrow's dip. Resolving that is arithmetic on your own finish time rather than a rule of thumb.
What helps, and what to stop spending money on
A short nap before the dip beats caffeine during it, and 20 minutes beats 45 because you wake before deep sleep rather than out of it. Bright light and getting outside help for as long as you are in it. What does not help is the supplement aisle. The timing of this dip moves with your body clock and with how much sleep you are short, so a nutrient cannot explain why it arrives two hours later for the person sitting next to you. If you are short three hours a night, that is the deficiency. This is a scheduling tool and not a clinician: persistent daytime sleepiness that survives a fixed schedule is worth taking to a doctor.
Common questions
+ − What causes the 2pm slump at work?
A second, shallower low in the body clock that sits about eight hours after the time you would wake with no alarm. Broughton described it as a circadian nap zone and Monk reviewed the performance evidence for it. The clearest proof that lunch is not the cause is that the dip still shows up in people who have eaten nothing. Food and a heavy meal can deepen it. They do not create it, and skipping lunch will not remove it.
+ − Is the 3pm slump real?
Yes, and so is the 2pm version and the 4pm version, which is the useful part of the answer. The dip is not pinned to a time on the wall clock. It sits about eight hours after the hour you would wake with no alarm, so a natural early riser hits it just after noon and a natural late type hits it in the late afternoon. Two people on the same roster can be at their worst two and a half hours apart, which is why the internet cannot agree on what time it happens.
+ − Why do I keep crashing at 4pm?
Most likely because your body clock runs late. The dip lands about eight hours after your unaided wake time, so the later you would naturally wake, the later it arrives. On an early start there is a second factor stacked on top: short sleep. Shifts beginning before 04:30 average about five hours a night, and sleep debt deepens the dip rather than shifting it. The fix is not caffeine at 4pm, which arrives too late to help. It is either a short nap before it or caffeine roughly 30 minutes ahead of it.
+ − Why do I get so tired driving home after work?
Because on an early roster the drive home and the afternoon dip often happen at the same time. Across 135 early-start rosters this tool models, the drive falls inside the dip on 53 percent of them, so more often than not. The shift ends, the task demand drops, the road is monotonous, and you are already at the low point. If that is your roster, the answer is to have taken the caffeine or the nap before you got in the car. Nothing you do at the wheel works quickly enough.
+ − What deficiency causes afternoon fatigue?
For the version described on this page, none, and it is worth saying plainly because a lot of supplements are sold against this question. The timing is the giveaway: the dip arrives at a different hour for different people, tracks the body clock, and deepens with short sleep, which is not how a nutrient shortage behaves. If you are sleeping five hours a night on a 4am start, the shortage is sleep. Genuine deficiencies do cause daytime fatigue, and iron, B12 and thyroid problems are all real. That is a blood test at a doctor's, not something a schedule or a supplement aisle can rule in or out.
What this is based on
- Broughton 1998, SCN controlled circadian arousal and the afternoon nap zone, Sleep Research Online 1(4):166-78
- Monk 2005, The post-lunch dip in performance, Clinics in Sports Medicine 24(2):e15-23
- Ingre et al. 2008, Sleep length as a function of morning shift-start time, Chronobiology International 25(2):349-58
- Sallinen and Kecklund 2010, Shift work, sleep and sleepiness, Scandinavian Journal of Work, Environment and Health 36(2):121-33
- Shift Work Hazards, NIH StatPearls
Related
ShiftDawn is a scheduling and education tool built on published circadian research. Not medical advice, not a diagnosis. If you suspect a sleep disorder, see a clinician.