Then there’s the hospital environment itself – many modern hospital buildings have small windows and dim indoor lighting that remains switched on day and night. This is problematic, because too little daylight and too much artificial light at night are disruptive to our biological rhythms and sleep.

Misaligned or flattened rhythms are a common feature of hospital patients. Compounding the problem, certain drugs, including morphine, can also alter the timing of circadian clocks, while patients’ sleep – also critical to their ability to heal – may be further disrupted by pain, worry or noise. It is leading to questions about how seriously this is impeding their recovery and survival.

Some of the strongest evidence comes from patients with heart disease. Like other tissues, the cardiovascular system has a strong circadian rhythm – our heart rate and blood pressure are lowest when we’re sleeping, but rise sharply upon waking up; our platelets, small blood fragments that help blood to form clots, are stickier during the day; while the levels of hormones like adrenaline, which constrict our blood vessels and make the heart beat faster, are also higher in the daytime. These circadian variations affect the timing of serious cardiac events, such as heart attacks.

“If you monitor people coming into emergency wards you find that heart attacks are most likely to happen between about 6am and noon compared to any other time of day or night,” says Martino. However, timing may also affect our ability to recover from heart injury.

One recent study suggested that for people undergoing heart valve replacement surgery, those who had surgery in the afternoon had half the risk of experiencing a major cardiac event during the following 500 days compared to those who underwent morning surgery. If all patients underwent afternoon surgery, this might result in one major event being avoided for every 11 patients, the researchers calculated. Other studies have indicated that, for patients recovering from a heart attack or heart surgery, those with greater exposure to daylight have higher survival rates and get out of hospital sooner.

Animal studies are providing insight into why this might be. When Martino and her colleagues exposed groups of mice either to normal or disrupted light-dark cycles after simulated heart attacks, they found significant differences in the number and type of immune cells that rallied to the heart, the amount of scar tissue – and, ultimately, survival rates. Mice whose circadian rhythms were disrupted, as they might be during a hospital stay, were more likely to die from their heart injury. Further studies have revealed differences in the type and number of immune cells that infiltrate injured heart tissue, depending on what time of day the injury occurs.

“Some intensive care units or cardiac care units will dim the lights a little bit at night, which is a bit helpful, but some don’t dim at all,” says Martino. “For example, if people come into emergency wards and there’s no beds available, they might be under bright light all night – or they might even be in a corridor all night, having had a heart attack or stroke. And so obviously their sleep and circadian rhythms are going to be profoundly disturbed during those first couple of days which are so critical for healing.”

So, what to do about it? Scheduling surgery for when the body is best placed to cope with it is one solution. For heart surgery, that might be the afternoon, but it may differ for other interventions. For instance, O’Neill’s study of wound healing suggested that more collagen is laid down when injuries are sustained during the daytime, which may be associated with greater scarring.