Sleep stages and cycle length
- National Heart, Lung, and Blood Institute (NIH).
How Sleep Works: Sleep Phases and Stages ↗Supports the 80–100 minute general cycle range, four-to-six-cycle context, three NREM stages, and the need for sensor measurements to classify stages.
Sleep duration by age
- Centers for Disease Control and Prevention.
About Sleep ↗Provides age-based daily sleep recommendations, including seven or more hours for adults ages 18–60.
- CDC / National Center for Health Statistics.
Short Sleep Duration and Sleep Difficulties Among Adults: United States, 2024 ↗Recent U.S. adult sleep-duration context and reference to the adult recommendation of at least seven hours.
Caffeine model
- Guest NS, et al. Journal of the International Society of Sports Nutrition (2024).
Common questions and misconceptions about caffeine supplementation ↗Reviews a mean caffeine half-life near five hours and broad individual variation, including physiological and environmental influences.
- National Academies / NCBI Bookshelf.
Pharmacology of Caffeine ↗Provides pharmacokinetic background for absorption and the wide range around caffeine’s average plasma half-life.
Editorial policy
We write “suggested,” “estimated,” and “may” when outcomes are uncertain. We do not claim that cycle timing guarantees refreshed waking, that clock times reveal REM sleep, or that a weekly shortfall can be exactly repaid.
Found an outdated source or a scientific problem? Contact hello@calculatingsleep.org with the page and supporting reference.