Slept four hours. Train or skip?

The baby was up at two, then again at four, and at some point you stopped pretending. Now it's seven, the plan says squats, and you're standing in the kitchen doing the maths on whether a session on four hours is training or just damage.

For a long time my answer was skip, and skip became a week, and a week became the months I wrote about in the story. So I'm biased toward going. But I've also been the guy grinding a top set at six in the evening on no sleep, and that wasn't training either.

What a short night actually does

Less to your top-end strength than you'd expect, and more to everything around it. Knowles and colleagues reviewed the sleep and strength research in 2018 and found that one bad night does less to a one-rep max than most people assume. What the studies more reliably showed was fewer reps at a given weight, worse coordination, and sets that feel harder than they are, especially later in the day. Craven's 2022 review of acute sleep loss found the hit grows later in the day and after more than one short night. The classic 1994 study by Reilly and Piercy, three nights of three hours each, saw submaximal lifts drop before maximal ones did.

So the honest summary is this. The bar will still go up. You'll get fewer good reps under it, your sense of effort will lie to you, and an evening session is worse than a morning one. A second and third short night cost much more than the first.

What a good trainer changes

They don't cancel. They take the top set off, because the top set is the one that needs you sharp. They cut a set from each exercise, keep the loads in a range you'd normally find easy, and put a cap on effort so nothing turns into a grinder. The pattern gets trained, the session ends early, and you're not paying for it on Thursday.

And they ask again tomorrow. One night is weather. A week of them is a problem no session design can fix.

What Stay Ready does with that morning

Today opens with the check-in: energy, sleep and soreness, each one to five, thirty seconds. On iPhone, last night's sleep comes in from Apple Health, and on Android from Health Connect. The sleep debt view keeps a rolling seven-day tally against an eight-hour target, paid down a little by any night you sleep past it.

The readiness number at the top of Today is built from that check-in (half of it), how fatigued your nervous system is from recent heavy work (a quarter), your joints (fifteen percent) and your most worked muscle (ten). Tap it and it shows every input and how far each one moved the number.

When readiness lands under 45, the launcher offers a Lighter session beside the planned one: one fewer set, loads at ninety percent, no top set, nothing past RPE 7. The example on the home page is the real arithmetic: four sets of eight at 200 lb becomes three sets of eight at 180. If the plan was written before the bad night, a banner on Today says what changed and why.

The number is honest about context. A four-hour night after two rest days, with fresh legs, often doesn't tip it, and the app offers the full session. The same night at the end of a heavy week, with the nervous-system score already low, does. The readiness simulator on this site runs the same formula if you want to move the sliders yourself.

Either way, Start session always works. The lighter session is advice, not a lock, and if you take the full one and log honestly, the remaining sets are re-planned from what you actually do.

What it won't do

It won't tell you to stay home, and it won't pretend four hours was fine. It also can't fix the week. Sleep debt stacks, the number keeps falling, and nothing in a session makes up for a third short night in a row. At that point the right call is the one your trainer would make: lighter again, and go to bed.

The Today chapter walks through the score and the launcher, and starting a session covers the check-in screen by screen. If you're not in yet, the waitlist is on the home page.