## TL;DR
Sleep is an active biological process essential for memory consolidation, metabolic regulation, and immune function. The circadian system orchestrates daily timing of sleep, hormone release, and gene expression.

## Core Explanation
Two-process model: Process S (homeostatic sleep pressure) and Process C (circadian alerting signal). Adenosine accumulation drives Process S — caffeine blocks adenosine receptors. Melatonin from the pineal gland signals darkness, promoting sleep onset.

## Detailed Analysis
Chronic sleep deprivation (<7 hours/night) associates with increased cardiovascular disease (45%), type 2 diabetes (30%), and all-cause mortality risk. REM sleep is critical for emotional memory processing; deep NREM consolidates declarative memories through hippocampal-neocortical dialogue.

## Further Reading
- NIH National Center for Sleep Disorders Research
- Sleep Foundation: Science of Sleep
- Nature Reviews Neuroscience: Sleep Special Collection