Sleep is not a passive recovery state — it is the most sophisticated memory consolidation process the brain deploys. Understanding sleep architecture — the four stages, their specific memory functions, and the physiological triggers that determine stage distribution — is the difference between sleep hygiene as generic advice and sleep optimisation as a precision protocol. This guide covers the N3 slow-wave mechanism for hippocampal-cortical memory transfer, what destroys it, and the 90-60-30 protocol that systematically addresses every major architectural disruptor.