The body clock calculator shows where your internal clock actually is – not where the wall clock says. From your usual sleep window it estimates your core-temperature minimum (CBTmin), your melatonin onset (DLMO) and your biological night, then maps them onto your destination’s time so you can see how far “out of phase” you will be when you land.
How does the body clock calculator work?
Your body runs on an internal ~24-hour circadian rhythm that does not instantly follow a new time zone. This tool takes the two anchors that define that rhythm and places them on a 24-hour dial. It estimates your core body-temperature minimum – the low point of alertness, roughly a couple of hours before your habitual wake time – and your melatonin onset (DLMO), the evening “start of biological night” a couple of hours before you normally fall asleep. Your chronotype nudges these anchors earlier or later.
After a flight, it keeps your clock where it was and overlays the destination’s local time, so you can read at a glance that “your body still thinks it is 3 a.m.” when the destination says it is breakfast time.
Which factors does it use?
| Habitual sleep window | Your normal bedtime and wake time – the backbone of the estimate. |
|---|---|
| Chronotype | Early, late or in-between shifts the whole rhythm; larks sit earlier, owls later. |
| Time-zone shift and direction | How far and which way your clock must move to catch up with local time. |
| Arrival time | Where local arrival falls against your biological night – the single most useful readout for planning that first day. |
The two anchors, CBTmin and DLMO, matter because they decide when timed light will help or hurt. Light after your temperature minimum pulls the clock earlier (good for eastward trips); light before it pushes the clock later (good for westward trips). That is the bridge to the light exposure planner.
What is CBTmin and why does it matter?
CBTmin is your core body-temperature minimum – the coldest, sleepiest point of your daily cycle, usually a couple of hours before you naturally wake. It is the pivot of the phase response curve: getting bright light just after it advances your clock, while light just before it delays your clock. Knowing roughly where your CBTmin sits is what makes a light plan work instead of backfiring, which is why this calculator estimates it first.
What does “biological night” tell me about arrival?
Biological night is the window when your body is chemically primed for sleep – melatonin up, temperature and cortisol low. If you land in the middle of it, you will feel wrecked and should not fight for a full day of activity; if you land near its edges, a well-timed dose of daylight can start the reset immediately. The calculator marks exactly where local arrival falls against your biological night, so you know whether to push through or ease in. For the underlying reason a flight causes this mismatch, see why does jet lag happen.
Frequently asked questions
Is my CBTmin measured or estimated?
Estimated. A true CBTmin or DLMO requires lab measurement. This tool infers a realistic position from your habitual sleep window and chronotype – a transparent estimate, not a clinical reading.
How is this different from the jet lag calculator?
The jet lag calculator gives an overall severity score and recovery estimate. The body clock calculator zooms in on where your inner clock sits and where it will be on arrival – the input a good light plan needs.
Does my chronotype really change the result?
Yes, modestly. An early type’s whole rhythm sits earlier, so biological night and CBTmin move earlier too, which can make eastward trips a little easier and westward trips a little harder.
Sources
- National Institute of General Medical Sciences (NIH) – “Circadian Rhythms” fact sheet – the body clock and its ~24-hour cycle.
- Lewy AJ et al. (1992), Chronobiology International – dim-light melatonin onset (DLMO) as a marker of circadian phase.
- Duffy JF & Wright KP (2005), Journal of Biological Rhythms – entrainment of the human circadian system by light.
- Sleep Foundation – educational overviews of the body clock and core body temperature rhythm.
CBTmin and DLMO are positioned by a transparent model from your sleep window and chronotype – not lab-measured, not a clinical figure.