Night shift workers face hidden health crisis as science explores biphasic sleep solutions

It is four in the morning on a hospital ward. A junior doctor has been standing for nine hours. Her body aches. Her eyes burn. When her shift ends at six and she finally reaches home, she cannot sleep. Her internal clock—shaped by millions of years of evolution to follow the sun—insists it is morning. Time to wake. Time to be alert. No blackout blind, no earplugs, no amount of darkness can silence it.

This is not weakness. It is a collision between the demands of modern work and the deepest machinery of human biology. More than three million people in the UK experience this collision every night: nurses, paramedics, engineers, lorry drivers, factory workers. They keep the country running while everyone else sleeps. And the scientific evidence about what this relentless battle costs them—in heart attacks, strokes, cancer, mental illness, and possibly their memories—has become impossible to ignore.

Sleep does far more than rest the brain and body. When we sleep, the brain consolidates memories, processes emotions, and solves problems that eluded us awake. It strengthens immune defences and repairs muscle tissue. Deep within the brain lies a plumbing system called the glymphatic system. Fluid runs along tiny channels beside blood vessels, washing away waste products that accumulate during waking hours. When sleep is disrupted, these toxins linger.

Researchers at Cambridge University analysed brain scans of more than 40,000 healthy people from the UK Biobank. They identified those whose drainage systems were struggling. The critical finding: those with the most impaired drainage were significantly more likely to develop dementia years later. Among the waste products cleared is amyloid and tau—the deposits that accumulate in Alzheimer's disease. A single sleepless night measurably raises amyloid levels in the fluid around the brain. Do that repeatedly, year after year, and the implications are troubling. A Swedish study tracking more than 13,000 shift workers for up to 41 years found that shift work in mid-life was associated with a 36 percent higher risk of dementia, with risk rising the longer someone worked shifts.

The damage extends beyond the brain. An analysis of 35 studies found that sleep reduced to around 4.5 hours for three or more nights significantly raised immune system activity. Normally protective, this chronic activation causes inflammation linked to heart disease. Disrupted sleep raises cortisol, the stress hormone, which promotes insulin resistance and pushes the body toward diabetes. Higher cortisol worsens sleep further, locking workers into a self-reinforcing cycle. The World Health Organization's cancer research agency has classified night shift work as probably carcinogenic to humans, citing evidence for links to breast, prostate, colon, and colorectal cancers—possibly because circadian disruption alters melatonin production, a hormone thought to suppress tumours.

But researchers are beginning to pinpoint what might help. In the Arctic Circle, where the sun barely sets in summer, a researcher named Line Victoria Moen studied shift workers wearing sleep-tracking rings. A pattern emerged. Many were not collapsing into one long sleep when they got home. They were sleeping in two distinct blocks—nine in the morning until one, then again in the afternoon before the next shift. Their bodies were forcing them to wake and recover additional sleep later. This pattern, called biphasic sleep, echoes something sleep historians have long noted: before artificial lighting, humans often slept in two blocks.

A historian at Virginia Tech spent 40 years in the archives of pre-industrial Europe and found that sleeping in two halves was the predominant pattern in the Western world until the mid-19th century. A typical family would go to bed around nine, wake naturally after midnight for prayer, chores, or conversation, then sink back into their second sleep. The pattern persisted until gas lamps began lighting London's streets in 1807, people stayed up later, and the gap between sleeps narrowed and eventually closed. A famous experiment gave volunteers 14 hours of darkness approximating a pre-industrial winter night. Within weeks, without instruction, they drifted naturally into sleeping in two halves.

Moen investigated 11,000 summaries of scientific papers to assess how prevalent biphasic sleep was among shift workers and what health outcomes were associated with it. She found almost nothing. Her full results are expected later this year. Several studies show that napping during shift work, where possible, is associated with reduced sleepiness and improved alertness. Even a 20-to-50-minute nap during or after a shift improves focus and reduces drowsy driving on the way home. What Moen wants to give shift workers is permission, backed by science, to stop fighting their body's signal and work with it instead. Her husband, a shift worker himself, always wakes after only three or four hours. There is no one home. It is still dark. His day rhythm drags him up. His body will not be overruled by a blackout blind. Clinical guidelines based on rigorous research could help millions like him make better choices.