Field notes
Sleep, researched properly
Everything here is written in the first person by one named human, checked against primary sources, and linked to those sources at the foot of the page. Where the evidence is thin, it says so — including when that is inconvenient for the calculator on the front page.
Start here
The three pieces the calculator is built on. Read them in this order if you are reading all three.
What a sleep cycle actually is, and why 90 minutes is a lie you can use
Four stages, wildly unequal shares of the night, deep sleep front-loaded and REM stacked at the end. Where the 90-minute figure came from, how wide the real spread is (70–120 minutes, with one large dataset putting the median at 110), and why I kept 90 in the calculator anyway.
Published 15 Aug 2026
How much sleep do I actually need, by age
The AASM and NSF numbers side by side, why they are ranges, where “eight hours” came from (a labour slogan, not a lab), the rare people who genuinely need less, and a two-week method for finding your own number.
Published 15 Aug 2026
Are sleep calculators accurate?
An audit of my own tool. What I will defend, what is educated guesswork, the claim the whole category makes that nobody has demonstrated, and the four inputs that matter which no calculator can see.
Published 15 Aug 2026
How this started
Less a reference page than an explanation of why the rest of them exist.
What you will not find here
Supplement affiliate links, mattress rankings, or a claim that a free web tool can fix your sleep. If something on this site turns out to be wrong, it goes in the corrections log with the date and what changed, rather than being quietly edited. The method behind every article is set out in the editorial standards, and what this site is for is set out in full on the about page.
The tool these notes are about
Two modes, adjustable cycle length, adjustable time to fall asleep, and every assumption printed on the page rather than buried.
Open the sleep calculator