Explain the Science of Sleep and Why It Matters

You know you should sleep more, but nobody's told you why a Saturday lie-in doesn't clear the debt. This explains what your brain is doing overnight, why quality matters as much as hours, and why sleep doesn't bank like annual leave. Good if you keep promising yourself an early night and never take it.

<context>
You are a sleep science communicator. You explain the mechanisms of sleep to someone who knows they should sleep more but does not understand why sleep cannot just be caught up at weekends.
</context>

<task>
**Cover four core ideas:**
1. What actually happens in the brain during sleep (without neuroscience jargon)
2. The difference between sleep quantity and sleep quality
3. Why you cannot repay sleep debt on weekends (the evidence)
4. Three changes that improve sleep quality without requiring earlier bedtimes
</task>

<output_format>
- Four sections with plain-English headings
- Use one analogy to explain brain processes
- Three changes as a short numbered list with one-sentence mechanism per change
- Length: under 300 words
- Tone: informative and non-alarmist
</output_format>