As Britain enters its fifth heatwave of the summer, many of us will by now be accustomed to waking up in floods of sweat.

But a top doctor has warned that this symptom should never be ignored – because it could be an early sign of cancer.

'During hot weather, almost everyone will experience warmer and more uncomfortable nights,' says Dr Jiri Kubes, a radiation oncologist at the Proton Therapy Centre at University College Hospital in London.

'That makes it very easy to overlook a symptom that might otherwise have prompted concern.'

According to Cancer Research UK, around a third (32 per cent) of people who noticed a potential 'red flag' cancer symptom did not contact their GP within six months.

These warning symptoms include night sweats, fatigue, persistent itching, change in appetite and bloating. While all are likely to be caused by other health issues and external factors, they can also be a sign of deadly cancer. 

And delaying diagnosis can have serious, and even fatal, consequences, says Dr Kubes. 

'The important distinction is whether there is an obvious explanation,' he said.  

'In the case of night sweats, if the room is cool but you are repeatedly waking with your pyjamas soaked through, that is different from simply feeling clammy on a hot night.' 

So when should waking up drenched in sweat be a cause for concern? 

Drenching night sweats are associated with certain types of blood cancer – a disease for which shockingly less than half of UK adults can name a single symptom. 

Blood cancers, such as lymphoma, are caused by changes in DNA within blood cells, which make them start behaving abnormally. These cells can keep multiplying, preventing healthy cells from growing and functioning. 

More than 40,000 people are diagnosed with the disease every year in the UK. 

While symptoms vary depending on the type of blood cancer, drenching night sweats is one of the most common symptoms along with fatigue and frequent infections. 

'One or two sweaty nights during a heatwave are rarely a reason to panic. However, if unexplained night sweats continue regularly for more than two or three weeks, it would be sensible to arrange an appointment with your GP,' Dr Kubes said. 

'People should seek advice sooner if the sweating is severe or appears alongside other concerning symptoms. 

'Crucially, you do not need to wait several weeks if you are feeling significantly unwell.' 

Other common symptoms include unexplained bruising, bleeding or pain in your bones and joints that interferes with your sleep or daily activities, unexplained weight loss, shortness of breath and itchy skin that doesn't improve with treatment. 

Painless swelling in the neck, armpit or groin are also a cause for concern.

However, Dr Kubes stresses that cancer is far from the most common explanation. 

'Night sweats have numerous possible causes, including menopause, anxiety, an overactive thyroid, alcohol and certain medicines such as antidepressants, steroids and some painkillers,' he said. 

'Likewise, tiredness during a heatwave is extremely common. It becomes more concerning when it's severe, prolonged and accompanied by symptoms such as drenching sweats, fever or weight loss.' 

Anyone experiencing these symptoms should contact their GP. 

Dr Kubes added: 'The message is not to panic after a couple of hot nights. It is to recognise a persistent and unexplained change, particularly when other warning signs are present. 

'Do not simply wait for the weather to cool if the sweating is severe, persistent or accompanied by other symptoms.' 

The UK has been hit with its fifth heatwave this year, with temperatures predicted to reach 38C (100F) in some areas today. 

England is currently under a UK Health Security Agency (UKHSA) amber heat health alert, which warns of a 'rise in deaths'. 

Deaths in Britain linked to extreme temperatures in May and June this year were almost double the entire figure for 2025, with 2,877 deaths linked to hot weather. 

'These figures highlight the significant danger that very hot weather can pose to people's health. This is particularly true for those who are more vulnerable including older adults and people with underlying medical conditions,' Dr Ross Thompson, Principal Environmental Public Health Scientist at UKHSA said.