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.
While there are many reasons for this phenomenon, one may be our circadian rhythm. This is the natural, internal process that regulates our sleep-wake cycle over a 24-hour period.
Not only does our circadian rhythm make us feel tired at night and alert during the day, it also regulates the timing of the body's processes – including digestion, metabolism, and appetite regulation – by secreting certain hormones based on what and when we eat, our physical activity and time of day.
Given how important the circadian rhythm is for our bodies and overall health, our team wanted to know what effect it has on our metabolism. 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).
Changes to these hormones could theoretically increase appetite while decreasing energy levels, leading to more calories eaten but fewer burned throughout the day. This could potentially lead to weight gain, but more research on this effect in humans is needed.
But given that all the studies were investigating different things (and so had different results), and they didn't measure changes in energy intake, expenditure and weight, this makes a suggested link between circadian rhythm disruption and weight gain inconclusive.
However, our study does find that the body's processes work best when you have regular sleep habits and don't ignore your circadian rhythm.
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.
However, it should be noted that not all studies agree that eating most of your day's calories in the morning leads to greater weight loss.
It has also been shown that higher levels of physical activity in people who eat breakfast (compared to those who didn't) may contribute to greater weight loss, so long as more calories are eaten in the morning instead of the evening.
Again, researchers aren't entirely sure why, but the theory is that morning feeding gives people more energy during the day, so they may be more active.
In contrast, consuming calories in the evening doesn't promote activity. Calories late in the evening may also disrupt circadian rhythms, leading to overall greater feelings of tiredness the next day and reduced physical activity.
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).
Research shows this appears to support weight loss predominantly through reducing calorie intake, likely because there's less time to eat. Intermittent fasting may also reinforce the natural circadian rhythm by stopping late-night eating.
While there's plenty of evidence supporting daytime eating as 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.