Sleep Quality Score: Predict Tonight’s Rest

Sleep is not a uniform state but a structured alternation between non-REM stages N1, N2 and N3 and REM sleep, repeating in cycles of roughly 90 to 110 minutes so that most adults complete four to six cycles a night. In healthy adults the night divides approximately into 5 percent N1, 45 to 55 percent N2, 13 to 23 percent N3 slow-wave sleep and 20 to 25 percent REM, but the distribution is not even across the night: deep N3 sleep is concentrated in the first third while REM periods lengthen toward morning, which is why a truncated night costs disproportionately more REM than deep sleep. Sleep timing and pressure are governed by two interacting processes, a homeostatic drive that builds with waking hours and is signalled partly by adenosine accumulation, and a circadian rhythm anchored by light exposure. The score here weights the inputs that shift those two processes: duration, bedtime relative to the 21:00 to 23:00 window, caffeine, evening screen light and daily exercise. The thresholds behind those weights are drawn from published guidance. A 2015 consensus panel convened by the National Sleep Foundation recommended 7 to 9 hours for adults aged 18 to 64, and the American Academy of Sleep Medicine and Sleep Research Society issued a joint recommendation of at least 7 hours the same year. Caffeine acts as an adenosine receptor antagonist with a half-life around 5 to 6 hours, so a 200 milligram dose taken at 15:00 still leaves roughly 100 milligrams circulating at 21:00; regulators commonly cite 400 milligrams a day as an upper limit for healthy adults. Evening light suppresses melatonin at intensities well below normal room lighting. The caveats are substantial: this is a habits questionnaire, not polysomnography, the stage percentages displayed are population averages rather than a measurement of your night, sleep efficiency and regularity are not captured, and untreated conditions such as obstructive sleep apnea can produce poor sleep despite ideal habits.

Objective sleep science began when Hans Berger recorded the human electroencephalogram in 1924 and published his findings in 1929, showing that brain electrical activity changed with state. In 1937 Alfred Loomis, E. Newton Harvey and Garret Hobart used EEG to divide sleep into a lettered series of stages, the first evidence that sleep progressed through distinct phases rather than deepening uniformly. The decisive discovery came in 1953 at the University of Chicago, where the graduate student Eugene Aserinsky and his supervisor Nathaniel Kleitman identified rapid eye movement sleep and its association with dreaming. William Dement, working with Kleitman, described the cyclical organization of the night in 1957 and established the roughly ninety minute cycle that pacing advice still relies on. Comparability required a scoring standard, provided in 1968 by the manual edited by Allan Rechtschaffen and Anthony Kales, which fixed the terminology of stages 1 through 4 and REM for nearly four decades until the American Academy of Sleep Medicine issued its revised manual in 2007, merging stages 3 and 4 into N3. The theoretical framework used to explain sleep timing, the two-process model separating homeostatic sleep pressure from circadian regulation, was proposed by Alexander Borbely in 1982. Public health interest followed epidemiological work through the 1990s and 2000s linking short sleep to cardiovascular and metabolic disease, prompting the duration recommendations published in 2015. Consumer estimation of sleep stages by wrist actigraphy and heart rate variability, which the stage percentages here echo, became widespread only in the 2010s and remains substantially less accurate than laboratory recording.