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.
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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.
First published 4 November 2026. Spotted an error? Tell us and we'll correct it. Read our editorial policy.