Free Tool

Caffeine cutoff: when was your last one, really?

Caffeine outstays its welcome. Work backwards from bedtime and find the hour you should have stopped.

Last one by
1:42 pm

Based on the two doses a 2023 meta-analysis reported thresholds for: 107 mg at least 8.8 hours before bed, 217.5 mg at least 13.2 hours. Everything between those is interpolated; outside them, flagged.

Why the number is bigger than you expect

Most people assume caffeine is done with them after a few hours. The measured answer is less forgiving. A 2023 systematic review pooling the controlled trials found that caffeine cut total sleep time by around 45 minutes and sleep efficiency by about 7% — and that to avoid losing sleep time at all, a standard 107 mg coffee needed to be drunk at least 8.8 hours before bed. A 217.5 mg pre-workout scoop needed 13.2 hours. For a 10:30 pm bedtime, that puts the coffee cutoff before 2 pm.

The part that should bother you: in a 2013 lab study, 400 mg of caffeine taken six hours before bed still cost participants more than an hour of objectively measured sleep — and they did not notice. Their own ratings of how well they slept showed nothing. If you have ever said "caffeine doesn't affect me", that is exactly the result you would expect to produce that sentence.

What this has to do with blood sugar

Sleep is one of the least glamorous levers on glucose control and one of the most reliable. Short or broken sleep shifts how the body handles glucose the following day — which is why a bad week of sleep can show up in readings that have nothing obviously to do with what you ate. We go through the evidence in Sleep and Blood Sugar.

Caffeine has a second, more direct line to your levels too, and it is more nuanced than the headlines: habitual coffee drinking tracks with lower type 2 diabetes risk in large cohort studies, while a single dose can blunt insulin sensitivity for a few hours in a lab. Both things are true. Coffee and Blood Sugar unpicks why they do not contradict each other.

Where this calculator is honest about its limits

It interpolates between two measured points. The review reported thresholds for 107 mg and 217.5 mg; this tool draws a straight line between them and extends it outwards, which is a reasonable estimate and not a measured curve. Outside that range the result is flagged for exactly that reason.

It also cannot know you. How fast caffeine clears varies several-fold between people — genetics, liver enzymes, smoking, pregnancy, some medications and oral contraceptives all shift it, and older adults tend to clear it more slowly. Treat the time it gives you as a starting hypothesis, not a prescription. The useful experiment is to move your cutoff earlier for a fortnight and see whether anything changes.

Educational tool only — not medical advice, and not a substitute for the guidance of your healthcare team. If you are managing a heart condition, an anxiety disorder, pregnancy, or taking medication that interacts with caffeine, your doctor's advice overrides any calculator. Nothing you enter here is stored or sent anywhere.

Sources

  1. Gardiner C, Weakley J, Burke LM, et al. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Med Rev. 2023. pubmed.ncbi.nlm.nih.gov/36870101
  2. Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195-1200. pubmed.ncbi.nlm.nih.gov/24235903

What now?

Sleep is one lever. The quiz finds the one doing the most damage in your particular week.

Take the villain quiz → All the tools