Caffeine

The late morning crash

Cortisol: A key hormone in the circadian rhythm

Cortisol is a steroid hormone produced by the adrenal glands in response to stimulation of the hypothalamic-pituitary-adrenal (HPA) axis, particularly via ACTH (adrenocorticotropin). It plays a central role in regulating circadian rhythm, metabolism, stress response, blood pressure, and blood sugar.

Cortisol upon waking: physiological peak

  • In a person with a regular sleep/wake rhythm, cortisol levels begin to rise about 30 to 60 minutes before spontaneous awakening.
  • This phenomenon is called CAR (Cortisol Awakening Response).
  • Peak cortisol levels typically occur 30 to 45 minutes after waking, with plasma concentrations increasing 50 to 100% above the nocturnal basal level.
  • It promotes wakefulness, increases alertness, blood sugar, and prepares the body for daytime activity.

Caffeine and cortisol: hormonal interaction

Caffeine is a psychoactive stimulant that works primarily by blocking adenosine receptors, an inhibitory neuromodulator responsible for the feeling of fatigue.

However, when caffeine is ingested at the time of the cortisol peak (for example, upon waking or within 1 to 2 hours), several phenomena can occur:

Redundant stimulation:
  • The body is already in its natural peak activation phase (cortisol).
  • Caffeine artificially adds stimulation via another axis (adenosine → dopamine/norepinephrine), causing an “overactivation” of the sympathetic nervous system.
Adaptive desensitization:
  • Regular caffeine consumption at the time of the cortisol peak could lead to a reduction in the endogenous cortisol response, through feedback mechanisms or receptor desensitization.
  • This can alter the natural cortisol cycle, making the person more dependent on caffeine to feel awake.
Rebound or “crash” effect:
  • By the time the caffeine effect wears off (after 3-5 hours), cortisol levels have already begun to naturally decline.
  • This gap between the withdrawal of stimulation and the drop in cortisol can cause an “energy crash” in the late morning, often interpreted as fatigue or a need for more coffee.

Conclusion

It's best to delay caffeine consumption for 90 to 120 minutes after waking up to avoid interfering with the natural cortisol cycle, avoid desensitization, and improve energy stability throughout the day. Drinking coffee first thing in the morning short-circuits the body's natural adaptive mechanism, which can affect morning alertness and coffee addiction in the long run.

Sources:
https://pmc.ncbi.nlm.nih.gov/articles/PMC2257922/
 https://pmc.ncbi.nlm.nih.gov/articles/PMC2249754/
https://www.thorne.com/take-5-daily/article/why-you-should-reconsider-that-morning-cup-of-coffee

Insomnia

Caffeine in the afternoon: a disruptor of deep and REM sleep

Caffeine, a stimulant of the methylxanthine family, acts primarily by blocking adenosine receptors, a key molecule in the regulation of drowsiness and the need for sleep (homeostatic pressure).

Adenosine: biological signal of fatigue

Throughout the day, adenosine accumulates in the brain, increasing feelings of fatigue and preparing the body for sleep. Its binding to A1 and A2A receptors decreases neuronal excitability and promotes sleep.

Caffeine: blocks sleep signals

In the late afternoon, if caffeine is ingested, it inhibits the perception of fatigue by preventing the action of adenosine. This mismatch between the actual need for rest and the perceived signal disrupts the sleep/wake cycle.

Physiological consequences

  • Prolonged sleep latency: the brain remains in artificial wakefulness mode.
  • Reduced deep sleep (N3): less physical recovery.
  • Fragmented REM: impact on memory and emotional balance.
  • Increased micro-awakenings: light and unrefreshing sleep.

Prolonged duration of action and individual variability

The half-life of caffeine varies between 4 and 10 hours depending on the individual (genetics, stress, hormones). Coffee consumed at 3 p.m. can still affect sleep quality at 10 p.m.

Conclusion

Drinking coffee in the afternoon interferes with natural sleep dynamics, masking fatigue signals and disrupting the structure of nighttime cycles. To maintain deep, stable sleep, consumption should ideally stop in the early afternoon.