Expose yourself to bright light first thing in the morning to stimulate cortisol secretion—the natural signal for alertness—by more than 50%, a hormonal ignition phenomenon that does not work in the afternoon.

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This claim is based directly on a clinical study (Leproult et al., 2001) conducted on healthy volunteers. Researchers observed that switching from dim light to bright light in the morning triggered an immediate increase of more than 50% in cortisol levels, whereas the same protocol in the afternoon had no impact. Other experimental work (such as the study by Scheer & Buijs, 1999) confirms that morning light effectively stimulates this wakefulness hormone. However, calling light the sole trigger is somewhat reductive, as the rise in cortisol naturally begins endogenously even before waking. External light exposure instead amplifies, optimizes, and stabilizes this biological rhythm. Finally, the expression 'in a few minutes' is slightly embellished, as the complete hormonal response physiologically occurs over an interval of 15 to 45 minutes.

the sources

1. 1Blume C, Garbazza C, Spitschan M 2019
2. 2Brown TM et al. 2022