Where our sleep numbers come from
Every sleep figure on SmartClock traces back to a public health page. This is the ledger: the source, what we rely on it for, which articles use it, and when we last checked it. All pages below were re-read on 2026-10-20.
What this site is and is not
SmartClock is a set of free clocks, timers and calculators. The sleep calculator does arithmetic: it counts back or forward in roughly 90 minute cycles from the time you give it. It is not medical advice, it does not diagnose anything, and the sleep articles are not written or reviewed by a clinician. They are written by Mark Fulton, a developer, from the public sources below. If you sleep badly or feel unwell, see a doctor.
Source ledger
| Source | What we rely on | Used in | Last checked |
|---|---|---|---|
| NHLBI (NIH): Stages of sleep | Sleep alternates between REM and three non-REM stages. One full cycle runs roughly 80 to 100 minutes, and most people get about four to six cycles a night. This is the basis for the cycle length in the sleep calculator. | 2026-10-20 | |
| NHLBI (NIH): How much sleep is enough | Recommended daily sleep by age: adults need 7 to 8 hours, teens 8 to 10, children 9 to 14 depending on age, and infants more, with naps counting toward the total for young children. | 2026-10-20 | |
| NHLBI (NIH): Healthy sleep habits | Keep the same sleep and wake times every day including weekends, keep the bedroom dim, quiet and cool, avoid big meals and alcohol late, and cut caffeine and nicotine well before bed. | 2026-10-20 | |
| CDC: About sleep | Adults 18 to 60 need at least 7 hours, teens 8 to 10. A steady bed and wake time and a quiet, cool room help, and feeling tired in the day despite enough hours can signal poor sleep quality. Adequate sleep is linked to better health outcomes. | 2026-10-20 | |
| Sleep Foundation: Is 6 hours of sleep enough? | Most research suggests six hours is too little for most adults, who generally need at least seven. People who habitually sleep six hours or less often feel adapted while their attention and judgment suffer. | 2026-10-20 | |
| Sleep Foundation: Sleep latency | Sleep latency is how long it takes to fall asleep. Healthy adults typically take about 10 to 20 minutes. Under about 8 minutes can suggest sleep deprivation, and over 20 to 30 minutes can point to trouble sleeping. This is why the calculator adds a fall-asleep buffer. | 2026-10-20 | |
| NIGMS (NIH): Circadian rhythms | Circadian rhythms are daily cycles driven by an internal clock that repeats roughly every 24 hours. Light is the strongest cue, with food, activity and temperature also playing a part. | 2026-10-20 | |
| CDC NIOSH: Sleep strategies for night work | For shift workers, a consistent daily sleep block works better than flipping between day and night sleep, short naps help when you can get them, and staying awake for very long stretches raises accident risk. | 2026-10-20 | |
| CDC Yellow Book: Jet lag disorder | Westward travel is generally easier to adjust to than eastward. Adjustment averages about 1.5 hours a day westward and about 1 hour a day eastward, and local daylight exposure matters. | 2026-10-20 |
Some articles also cite individual studies. The rows above are the public-health pages the numbers lean on most.
Changelog
- 2026-10-20: Page created. Nine public sources re-read and summarized here.
Try the sleep calculator or read is 6 hours of sleep enough.