It is well documented that people who eat late at night tend to be heavier than those who do not. But is it meal timing or something else that causes them to pack on the pounds?

Dozens of factors can be involved when a person becomes overweight or obese. Yet, while it is difficult to point to a single factor, scientists noticed such a strong association between late-night eating and weight gain that they decided to investigate the relationship further.

The obvious question to address first is whether late-night eating is simply adding extra calories to an already-full day of eating. If this were the case, there would be nothing particularly special about nighttime eating that makes people more prone to weight gain (except maybe a bigger appetite).

One study found that while nighttime eaters do tend to eat more calories over the course of the day, this phenomenon was a symptom, rather than a cause of meal timing. What the researchers found is that free-living people who ate more of their food earlier in the day were less likely to overeat later than people who ate less in the morning. Eating at night was less satisfying and therefore more food was consumed overall. So while total calories may indeed play a role in the link between nighttime eating and weight gain, meal timing seems to be important, as well.

The reason food timing depends on more than just calories has to do with your body’s circadian rhythms, or biological clock. Most animals, plants, and microorganisms have internal systems that adapt our bodily functions to sync with light-dark cycles. In animals, including humans, these systems depend largely on hormonal signals, and these determine when we sleep, when we eat, and how our bodies respond to things like food (i.e. your metabolism).

Research has shown that eating at night as opposed to during the day results in lower glucose tolerance and insulin resistance. These problems are compounded when more food is eaten at night. Such metabolic changes are associated with weight gain – and indeed, mice that are only allowed to eat during their usual sleep cycle gain more weight than those who can only eat during their active cycle. Hormonal changes may also explain why food is less satisfying in the evening than during the day.

Late-night eating should thus be avoided, and as explained above eating breakfast earlier in the day can make this easier. Since circadian rhythms are tied to light cycles, you should also avoid bright light exposure late in the evening. Mice who were subjected to even dim lighting during their sleep cycle were more likely to eat at inappropriate times and gain weight than mice on a regular light-dark cycle – despite consuming the same total number of calories and getting the same amount of exercise.

To summarize, your metabolism is closely tied to circadian rhythms and eating late at night can cause weight gain if it becomes a habit.

When it comes to the science of wellness, distinguishing the facts from the urban legends can be tough. That’s why we’ve enlisted Darya Pino – a scientist, foodie, and self-proclaimed geek girl. Check out the ZocDoc Blog every Tuesday to see her bust the biggest myths in health.

Image: 40+137 Snack, a Creative Commons Attribution (2.0) image from Bark’s photostream.