# How Sleep Facts grades the evidence

Source: https://sleepfacts.uk/about/evidence-grades/
Author: Tess Marlow · Updated 2026-11-04

**Short answer:** Each health claim gets one of four grades. Strong means good randomised trials, or a review of them, agree. Moderate means trials are small or few, or large long-term studies agree but can't prove cause and effect. Weak means small, short or mixed studies. None means no sound human study supports the claim.

## What do the four grades mean?

| Grade | What it means | What would move it up |
| --- | --- | --- |
| Strong | Several good randomised trials, or a solid review of them, agree. For links between sleep and disease: very large long-term studies backed by trials showing the mechanism | Already the top grade |
| Moderate | Trials point the same way but are small or few, or large long-term studies agree but can't rule out other explanations | Larger, longer or independent trials |
| Weak | Small or short studies, surveys, or results that disagree with each other | Consistent results from well-run trials |
| None | No sound human study supports the claim, or the good studies found no effect | Any well-run human trial with a positive result |

The dots beside each grade fill up with it: three for strong, two for moderate, one for weak and none for no good evidence.

## Why is sleep research hard to grade?

Because the most important questions can't easily be tested. Nobody can randomly assign thousands of people to sleep five hours a night for twenty years. So the evidence that short sleep harms the heart or raises the risk of diabetes comes from long-term studies that follow people, and those always leave room for doubt: people who sleep badly may already be unwell, stressed, poorer or working night shifts.

To get past that, we look for two other kinds of evidence:

- **Laboratory trials** that restrict healthy volunteers' sleep for days or weeks and measure what changes, such as blood pressure, appetite hormones or blood sugar control.
- **Treatment trials** that improve people's sleep, for example with CBT for insomnia, and see whether something else, such as mood, improves as a result.

When long-term studies, lab trials and treatment trials all point the same way, we can be fairly confident. When there are only long-term studies, the grade is lower.

## What do we look at in each study?

- **Was there a fair comparison?** A placebo group matters a lot for sleep aids, because people often sleep better simply because they expect to.
- **How was sleep measured?** Sleep diaries and questionnaires are useful but less reliable than measuring sleep in a lab or with a wrist monitor.
- **How many people, and for how long?** Small, short trials can find big effects by chance.
- **Who paid for it?** We note when a manufacturer funded or ran a trial.
- **Has it been repeated?** One trial, however good, is a starting point.