- Circadian rhythm
- The roughly 24-hour cycle your body runs on its own. Left alone it drifts a little long, which is why it needs a daily cue — mostly light — to stay lined up with the world outside.
- Zeitgeber
- German for "time giver": any outside signal that tells your internal clock what time it is. Light is the strongest one. Meal timing, movement, and social schedules are weaker ones.
- Suprachiasmatic nucleus (SCN)
- A small cluster of cells in the brain that acts as the master clock. It takes its instruction from the eyes and sets the timing for sleepiness, temperature, and hormone release.
- Sleep pressure
- The buildup that makes you sleepier the longer you've been awake. It starts accumulating the moment you get up and only clears properly during sleep.
- Adenosine
- The molecule most closely associated with that buildup. It accumulates through the waking day and is what caffeine temporarily blocks the sensation of.
- Post-lunch dip
- A stretch in the early-to-mid afternoon where alertness sags before climbing again in the early evening. It shows up in people who skipped lunch entirely, so lunch isn't really the cause.
- Half-life (caffeine)
- How long it takes your body to clear half of what you drank. For most adults it's several hours, which is why an afternoon cup can still be in the room at bedtime.
- Social jet lag
- The gap between the schedule your body wants and the one your calendar imposes — most visible when weekend sleep timing drifts hours later than weekday timing.
- Sleep inertia
- The groggy stretch right after waking, when you're technically up but not yet running. It usually fades within the first half hour or so, faster with light and movement.
- Electrolytes
- Minerals like sodium, potassium, and magnesium that carry a small charge. They help fluid stay where it's useful and are part of why water alone doesn't always do the job.
- NEAT
- Non-Exercise Activity Thermogenesis — the energy spent on ordinary movement: walking to the shop, taking the stairs, standing up. Across a day it often outweighs a formal workout.
- Wind-down
- The stretch before bed where your body hands off from awake to asleep — temperature drifting down, attention loosening. It takes time, and it doesn't care what time you've decided to be asleep.