Set your cortisol rhythm: bright light within the hour after waking to spike morning cortisol, low cortisol at night — and, contrary to popular belief, caffeine in a regular drinker and cold exposure done twice a week or more supposedly don't raise cortisol.

noli's read

The core mechanism — a cortisol spike in the hour after waking, the cortisol awakening response (CAR) — is well documented: several controlled studies (Leproult et al., Scheer & Buijs, published in endocrinology journals) show morning bright light raises this spike by roughly 35-50% compared with dim light, an effect that fades if the same light is given in the evening. The caffeine claim is optimistic: a controlled study (Lovallo et al., Psychosomatic Medicine) found daily use blunts the cortisol response to caffeine but doesn't eliminate it — tolerance is partial, not complete, and a more recent study even links habitual caffeine use to heightened cortisol reactivity under stress. The cold-exposure claim is missing the same nuance: cold water immersion does raise cortisol acutely (studies at 10-14°C immersion), and it's only after weeks of repeated exposure that this response fades through habituation — saying cold "doesn't increase" cortisol conflates the acute response with the long-term habituated one. The light/rhythm advice is the strongest of the three; the caffeine and cold claims turn nuanced biology into an absolute rule. Verdict: mixed — the underlying science is real, but some claims outrun what the cited research shows.

the sources

1. 1Pandi-Perumal SR, Spence DW, Srivastava N, et al. 2022
2. 2Dewar M, Alber J, Cowan N, Della Sala S. 2014
3. 3Weng L, Yu L, Rong J, et al. 2025