Having the occasional bad night’s sleep isn’t anything to worry about in and of itself, the NHS says.

But if the issue lasts a long time or starts to affect your day-to-day life, it could be worth speaking to a doctor, as this might be down to conditions like insomnia.

Still, those terms can be a little tough to navigate. How long is “a long time”? It feels like everyone complains about feeling tired – how can we tell “normal” fatigue from sleep-disorder-level exhaustion?

Here, doctor and Fellow at the Royal College of Anaesthetists, Dr Sunny Nayee, shared the “3-3-3 rule” he uses to tell bad sleep from a more lasting issue.

What is the “3-3-3 rule”?

“If you experience disrupted sleep at least three nights a week for at least three months, medical practitioners no longer regard it as lifestyle related but in the........

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