Typical Circadian Cycle Calculator (Hours)

The circadian cycle and chronotypes

The circadian cycle is an internal biological rhythm whose period sits close to 24 h. Measured in temporal isolation it usually runs a touch longer, around 24,2 ± 0,2 h. The suprachiasmatic nucleus (SCN) in the hypothalamus generates it, and the body keeps it locked to the 24-hour day mostly through light hitting the melanopsin-containing ganglion cells in the retina.

In a typical adult, cortisol peaks between 7–9 a.m. and melatonin begins to rise around 9 p.m. (the DLMO, or Dim Light Melatonin Onset), while core body temperature bottoms out near 4 a.m. Your chronotype slides those markers around. Morning types ("larks") hit their peaks 1–2 h earlier, and evening types ("owls") run 1–3 h later. Back in 2017 the Nobel Prize in Physiology or Medicine went to Hall, Rosbash and Young for uncovering the per and tim clock genes behind the transcription–translation feedback loop that powers all of this.

Applications

It comes up when planning shift work and rotating schedules, managing jet lag with light therapy and well-timed melatonin, picking surgery or chemotherapy windows (chronopharmacology), lining up study sessions or peak performance with your chronotype, and timing meals for metabolic health (chrono-nutrition).

FAQ

How do I find my chronotype? The MEQ (Horne-Östberg) and MCTQ (Munich) questionnaires both produce a score. If you want a quick stand-in, take the midpoint of your sleep on free days and correct it for any sleep debt.

Why does jet lag feel worse going east? Since the internal clock runs a little longer than 24 h, pushing it later (heading west) is easier than dragging it earlier (heading east). Expect roughly 1 day of adjustment for each time zone you cross.

Does blue light at night really matter? It does. Short-wavelength light (450–480 nm) above 100 lux for 1–2 h suppresses melatonin and pushes sleep onset later. Keep the lights dim and warm after sunset and your DLMO stays intact.