Caffeine's half-life
Caffeine has a half-life of approximately 5–7 hours.
This means that if you drink a coffee at 3pm, half of that caffeine is still active in
your system at 8–10pm. A quarter of it is still present at midnight.
Most people dramatically underestimate how long caffeine stays in their system.
They drink a 3pm espresso, feel fine by evening, and then wonder why they can't
fall asleep at 11pm. The answer is often: the caffeine.
A practical rule that works well for most people: no caffeine after 1–2pm.
If you are particularly sensitive to caffeine, or if your sleep is badly disrupted,
consider cutting the deadline back to noon or even earlier.
Hidden caffeine sources
Caffeine appears in more places than most people realise:
Obvious: coffee, espresso, energy drinks, pre-workout supplements.
Less obvious: tea (including green tea, matcha), dark chocolate,
many soft drinks, some medications (including certain pain relievers and cold remedies),
decaffeinated coffee (which typically contains 10–30% of a regular cup's caffeine).
If you have been drinking "decaf" in the evenings and still sleeping poorly,
this may be a contributing factor.
Alcohol: the sleep thief
Alcohol is sedating, which is why many poor sleepers use it as a sleep aid.
It does help you fall asleep faster — but it significantly degrades sleep quality
in the second half of the night.
As your body metabolises alcohol (usually 3–5 hours in), it produces a rebound effect:
increased alertness, fragmented sleep, suppression of REM sleep, and more vivid dreams
or nightmares. You may "sleep" for 8 hours and feel exhausted.
The recommendation is not necessarily abstinence — but to understand that alcohol
is not a sleep aid. It is a sleep disruptor wearing a sedative costume.
Why tracking matters
The sleep diary you started in lesson 1 is more than a record —
it is the primary tool we will use to calculate your sleep efficiency:
the percentage of time in bed that you are actually asleep.
Sleep efficiency = (Total sleep time ÷ Time in bed) × 100
A healthy sleeper typically has a sleep efficiency of 85% or above. Many people
with insomnia are at 60–70% — spending large amounts of time lying awake in bed,
which (as you now know) reinforces conditioned arousal.
In the full course, we use sleep efficiency to calibrate sleep restriction therapy —
the most powerful CBT-I technique. For now, continue your diary and we will look
at your numbers in lesson 5.