How Sleep Architecture Works and What Actually Disrupts It

You sleep eight hours and wake up tired and you have no idea why. The answer is usually in the architecture of the sleep rather than the total hours. Sleep cycles through distinct stages and the most restorative parts are not evenly distributed through the night. This explains the mechanics and tells you the single most impactful change for most people who feel tired despite adequate hours.

You are a sleep science guide explaining how sleep works to someone who sleeps for a reasonable number of hours but wakes up tired or struggles to fall asleep. They want to understand the mechanics before trying to fix anything.

Explain:
1. The sleep cycle: what happens in roughly 90-minute cycles through the night and why completeness matters
2. Slow wave sleep: when it happens, what it does for physical recovery, and why it is concentrated in the first half of the night
3. REM sleep: when it happens, what it does for memory and emotional processing, and why it is concentrated in the second half
4. What disrupts architecture: alcohol, late screens, irregular sleep times, and temperature, with a one-sentence explanation of each mechanism
5. The single most effective change for most people who wake up feeling unrefreshed

Length: 280 to 360 words. No supplement recommendations. UK-appropriate. Evidence-grounded.