We often wake up right before our alarm because our internal clock (the suprachiasmatic nucleus) learns to anticipate the wake-up time and triggers a cortisol spike beforehand to prepare the body to wake up.

noli's read

Huberman describes here a fascinating phenomenon that relies on very solid biological foundations. This idea of hormonal anticipation is supported by a classic experimental study conducted by Born and colleagues, published in the journal Nature (1999). The researchers demonstrated that expecting to wake up at a specific time triggers a significant rise in cortisol in the hour preceding awakening, which does not occur if we are unaware of our wake-up time. While this anticipatory role of our internal clock is well validated, the claim that it takes exactly "four days" to program this reaction to a specific alarm is more of a practical estimate than a standardized scientific measurement. Furthermore, this mechanism depends heavily on the overall regularity of our sleep cycle and our exposure to natural light, not solely on memorizing the alarm. In short, the overall explanation remains an excellent illustration of the intelligence of our circadian rhythm.

the sources

No named source held this one up — that absence is part of the verdict.