ALEX JOHNSTONE & LEONIE RUDDICK-COLLINS, THE CONVERSATION 20 AUGUST 2020 Most diet and health advice is broadly based on the assumption that a calorie is a calorie (and it doesn't matter when they're consumed). But some research suggests that our bodies actually use calories more efficiently when consumed in the morning as opposed to the evening. This points to a strategy that could be beneficial for weight loss. We conducted a review that examined studies in humans whose circadian rhythms had been disrupted on purpose by researchers, or because of night eating syndrome, where a person ate more than 25 percent of their daily calories in the evening or middle of the night. Based on these studies, it was clear that our bodies do indeed prefer us to eat during daylight hours – in sync with our natural circadian rhythm. Most of the studies showed that intentional circadian rhythm disruption and night eating both caused changes to many important hormones that regulate appetite, energy expenditure and glucose regulation (resulting in changes in the levels of circulating insulin, leptin, cortisol and other appetite hormones in the blood). Metabolism and body weight Other studies have also found evidence that suggests time of day influences energy balance and body weight. For example, eating more calories in the late evening has been linked with weight gain and obesity, possibly because of lower appetite regulation in the evening, or because late meals disrupt circadian rhythms and our energy levels – making us less likely to exercise the following day. Eating most of your calories in the morning may also lead to greater weight loss. This weight loss seems to occur despite similar daily food intake and activity levels to those who ate more calories in the afternoon or evening. Though it's not known why this is the case, it may be because people who miss breakfast snack more in the evening – or it could be because later food intake disrupts circadian rhythms. A recent study also found changes in the brain's signals that control food reward in response to feeding time. The researchers think that eating more calories in the morning may improve body weight by reinforcing the brain's reward centres related to food – therefore reducing overeating. Time-restricted feeding (sometimes known as "intermittent fasting") is another approach gaining interest. This is when people are only able to eat within a specific timeframe over the day (such as over an eight or 12-hour period).
While there's plenty of evidence supporting daytime eating a s it's more in line with our natural circadian rhythm, more research is needed to fully understand the effect that this has on body weight. Of course, the type of foods you choose and your portion sizes have the biggest impact on your health. But if it's the case that eating time is linked to differences in body weight and health, then when you eat may also need to be included in dietary advice.