Circadian Rhythm Calculator
Finding Your Natural Chronotype
Left to its own schedule — no alarm, no obligations — everyone's body settles into a preferred sleep window. Some people naturally drift toward early nights and early mornings; others toward late nights and late mornings. This tendency is called chronotype, and one simplified way to estimate it is by finding the midpoint of a person's unforced sleep window: the exact clock time that falls halfway between natural bedtime and natural wake time.
The Formula
This mirrors the core idea behind the Munich Chronotype Questionnaire (MCTQ), a widely used research approach that estimates chronotype from the midpoint of sleep on days without an alarm. Where that midpoint falls on the 24-hour clock places you into one of five broad chronotype categories, from definite morning type to definite evening type.
Where This Matters
- Scheduling demanding work — aligning focus-heavy tasks with your natural energy window, rather than fighting your chronotype, tends to work better than a one-size-fits-all schedule.
- Explaining a mismatch with work hours — a pronounced evening chronotype forced into an early start is more prone to chronic sleep debt than someone whose chronotype already matches the schedule.
- Choosing a more compatible bedtime — the calculator's suggested ideal window gives a starting point that respects your natural tendency rather than an arbitrary bedtime.
Chronotype Categories by Sleep Midpoint
| Sleep midpoint | Chronotype | Suggested window |
|---|---|---|
| 1:00–3:29 AM | Definite Morning Type (Lark) | Bed ~9–10 PM, wake ~5–6 AM |
| 3:30–4:29 AM | Moderate Morning Type | Bed ~10–11 PM, wake ~6–7 AM |
| 4:30–5:29 AM | Intermediate Type | Bed ~11 PM–12 AM, wake ~7–8 AM |
| 5:30–6:29 AM | Moderate Evening Type | Bed ~12–1 AM, wake ~8–9 AM |
| 6:30 AM and later | Definite Evening Type (Night Owl) | Bed ~1–2:30 AM, wake ~9–10:30 AM |
How to Use This Calculator
- Enter the bedtime you naturally gravitate toward with no alarm involved, in the Natural Bedtime field.
- Enter the time you naturally wake up without an alarm, in the Natural Wake Time field.
- Select Calculate to see your sleep duration, sleep midpoint, and estimated chronotype.
Related Calculations
Once you know your chronotype, plan cycle-aligned sleep and wake times with the Sleep Cycle Calculator, or check age-appropriate duration with the Sleep Need Calculator.