Sarah (our protagonist from the 4:17 PM dilemma) decided to test her limits. It was Saturday, and she was heading to a 7 PM movie. "It's just a small cup of tea," she told herself, "it's mostly water!" She fell asleep easily at midnight, but Sunday morning felt like she'd run a marathon in her sleep.

Achieving "Zero" caffeine by bedtime is the holy grail of sleep hygiene. Why? Because even 10mg to 20mg of caffeine can reduce your Deep Sleep (Slow Wave Sleep) by up to 20%.

The Long Tail of Decay

Caffeine decay is exponential, not linear. It doesn't leave your system at a steady rate; it halves, then halves again. To go from a modest 100mg to nearly zero (< 5mg) takes about four to five half-lives.

The Math: 5 hours x 4 half-lives = 20 Hours.

This means if Sarah wanted zero caffeine in her brain at midnight, her last cup would have had to be at 4 AM that same morning! While that might be too extreme for most, understanding this decay helps you set a more realistic "Low Impact" threshold.

The Outcome: Sarah realized that her "small cup of tea" (approx. 30mg) still left about 15mg active when her head hit the pillow. It didn't keep her awake, but it stole her recovery. Now, she sticks to caffeine-free herbal blends for her evening movie dates.