Britain’s electricity system operator issued a rare summer appeal for more generation on Wednesday, as the country’s third heatwave of the year tightened power margins and a new survey showed more than 15 million people suffered health problems during the recent hot spell. The National Energy System Operator (Neso) forecast a tight period for Thursday evening, citing “extreme temperatures” across Europe that reduce the efficiency of nuclear, gas and water-cooled plants at the same time as demand for fans and air conditioning surges. The margin notice, in effect from 6.30 p.m. to 10.30 p.m., is the third such call this year—a measure more typically deployed during winter heating peaks—and signals the operator’s desire for a larger safety cushion, though it does not imply blackouts are imminent.

The strain on the grid mirrors a broader public-health burden. A survey by the End Fuel Poverty Coalition, a grouping of more than 100 anti-poverty, health and housing organisations, found that 28 percent of British adults felt unwell because of the heat. The impact was sharply unequal: 47 percent of those in financial difficulty and 44 percent of disabled people reported health problems. Separately, a YouGov poll of 2,135 adults for Greenpeace documented mass sleep deprivation, with two in three people struggling to sleep and almost half losing at least three hours a night. A scientific review published in January found that for every 1°C rise in night-time temperatures, participants lost between seven and 14 minutes of sleep, with worse outcomes among the elderly, low-income households and those in poorly ventilated housing.

Public health researchers at University College London and NHS clinicians point to a gendered dimension of heat vulnerability. Women produce less sweat and begin sweating at a higher temperature, while natural hormone fluctuations during menstrual cycles, perimenopause, pregnancy and breastfeeding can impair the body’s thermoregulation. Socioeconomic factors compound the biological risk: women are more likely to be lower paid, to act as primary caregivers and to live longer, which increases exposure to heat stress and conditions such as dementia that blunt thirst perception. Air ambulance crews in southern England and the South Western Ambulance Service both reported a surge in call-outs during the June heatwave, ranging from major trauma to dehydration and sunburn.

The heat is forecast to peak on Thursday and Friday, with temperatures widely exceeding 30°C and possibly reaching 36°C in parts of England. Neso’s margin notice for Thursday evening will be closely watched: the two previous notices issued in June were cancelled ahead of time, indicating the operator was satisfied with supply levels. Whether this third call is also withdrawn will offer a real-time gauge of the grid’s ability to cope with summer heat stress, as health authorities maintain warnings and the coalition urges better protection for those unable to afford cooling.