What CBTmin is, and why light timing flips around it

Updated 2026-08-17

CBTmin is your core body temperature minimum — the coldest point of your night, about two hours before you normally wake. Wake at seven and yours sits near five in the morning. You will never feel it happen, and it decides whether light helps you or hurts you.

The rule that flips

Light in the hours after CBTmin pulls your clock earlier: sleepy sooner that night, awake sooner the next day. That’s what you want flying east, or becoming an early riser.

Light in the hours before CBTmin pushes your clock later. That’s what you want flying west, or recovering from a schedule that starts too early.

Same lamp, same window, same twenty minutes — opposite results either side of one moment.

Where the trap is

CBTmin travels with your body, not with your alarm clock. When you start shifting earlier, your alarm moves an hour a night; your CBTmin follows more slowly. Push the alarm too hard — two or three hours in one jump — and you’ll be waking before your CBTmin, in the delaying zone.

Then the person doing everything right, curtains open, straight out into the morning, is quietly moving themselves west while trying to go east. They conclude that light therapy is nonsense. It isn’t; the timing was inverted.

How to keep on the right side of it

Move in steps your body can follow. About an hour a night. Not three.
Track where CBTmin is today, not where it was at home. It moves as you do.
When you must be up before it, keep the light low until it’s passed, then get as much as you can.

A worked example

You wake at 07:00 normally, so CBTmin ≈ 05:00. You’re flying east and shifting an hour a night:

Night 1 — bed 22:00, wake 06:00. CBTmin has moved to about 04:40. You wake well after it: morning light advances you. Good.
Night 4 — bed 19:00, wake 03:00. CBTmin is around 03:20. You’re now waking before it. Bright light at 03:00 would delay you, undoing the night.

At that point the instruction has to change from get outside to keep it dim until 03:20. Ray makes exactly this switch, which is why some mornings its cue is “keep it dim for now” rather than “get bright light”.