Caffeine blocks adenosine receptors but does not clear adenosine. When caffeine is metabolised, adenosine floods in — the "caffeine crash." The sleep debt is still there; the signal was just suppressed.
Half of a 3 pm coffee is still active at 10 pm. This has dramatic consequences for sleep quality even when sleep onset is unaffected. Light and deep sleep stages are disrupted by residual caffeine.
A common misconception. Melatonin tells the body night has begun and coordinates the timing of sleep, but it is not a sedative. Taking melatonin supplements does not generate deeper sleep — it shifts the clock.
The body's internal clock lags behind the new time zone. Eastward travel is harder than westward because it requires advancing the clock (harder biologically than delaying it). Recovery typically takes one day per time zone crossed.
Whether you are a morning or evening person is not purely habit — it is influenced by genetics and shifts predictably across the lifespan (adolescents skew late; older adults skew early).
Given the half-life, a cutoff in the early afternoon ensures minimal caffeine remains active at sleep time. Adjust based on individual metabolism.
Low-dose melatonin (0.5 mg) taken at the target bedtime in the new time zone for three to four days helps reset the clock. It is not a general sleep supplement.
If you are an evening person, scheduling deep focus work at 7 am is fighting your biology. Identify your peak alertness window and protect it.
On eastward flights, avoid morning light on arrival for the first day; seek afternoon light instead to ease the clock forward gradually.
Tea, energy drinks, and dark chocolate all contain caffeine. Build a complete picture of afternoon caffeine intake, not just coffee.
Chapter 2 is the most practically useful chapter in Part 1 — it translates the biological model from Chapter 1 into concrete, actionable recommendations about caffeine and jet lag. The melatonin section is valuable for correcting a widespread misunderstanding (people treat it as a sleep drug when it is a timing drug).
Walker's tone remains urgent. The caffeine half-life data is well-established and actionable immediately.
"The recommended 'dose' of melatonin for jet-lag purposes is far lower than what is sold over the counter in most countries." (p. 38)