33% of Brits report feeling tired about a third of the time, while 13% say they’re exhausted 24/7, YouGov reports.

The NHS says “A few sleepless nights are usually nothing to worry about, but it can become an issue if a lack of sleep starts to affect your daily life.”

But in what feels like a culture of constant low-level, never-ending tiredness, how are we meant to tell when “normal life” ends and “signs of chronic fatigue” begin?

That question might be part of the reason why the Royal College of Surgeons says 90% of chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), cases are thought to go undiagnosed.

So, we spoke to Dr Milli Raizada, GP and owner of Dr Milli Health about how to spot CFS from “normal” fatigue.

“Prolonged, unrelenting fatigue lasting more than six months meets the diagnostic criteria for CFS/ME, but even tiredness that persists for a few weeks should not be ignored,” Dr Raizada told HuffPost UK.

If possible, she adds, you should see a GP before you hit the six-month mark as “early assessment and blood tests can help identify or rule out potential causes.”

Then, there are the symptoms to consider.

“Fatigue that worsens after even mild activity, known as post-exertional malaise, is a hallmark of CFS/ME and is often overlooked,” the GP shared.

“Other symptoms that may accompany persistent fatigue include cognitive difficulties such as memory problems and poor concentration, un-refreshing sleep where an individual wakes up exhausted despite adequate rest, muscle or joint pain, unexplained headaches, dizziness or heart palpitations, which may indicate dysautonomia or POTS, and frequent sore throats or swollen glands, suggesting immune dysfunction.”

Whatever else is going on with your body, the doctor says, constant exhaustion should never be ignored.

CSF is only one cause for never-ending exhaustion, Dr Raizada told us.

Other conditions, like “thyroid disorders, anaemia due to low iron, B12, or folate, post-viral fatigue,” and, “In rare cases, a more serious condition… such as occult cancer” can cause similar symptoms.

So, she advises, “Certain red flag symptoms require urgent medical attention rather than waiting for a routine GP appointment.

“These include unexplained weight loss, abnormal bleeding, persistent lumps, new rashes, or any other concerning physical changes.”

It can be helpful to keep a journal to track your symptoms and find possible causes, the GP continued.

Your doctor might recommend a blood test and/or lifestyle changes when you see them.

“If symptoms continue beyond a few weeks or if many of the symptoms mentioned are present, seeking medical advice promptly is essential,” Dr Raizada ended.