Late-night snacking is seen as a big no-no for most people. A review of five randomized controlled trials sheds a bit more light on this issue, suggesting that munching in the morning or at night may not matter as much as some people think (at least with regard to weight gain).
First, let’s rewind back to the halcyon days of 2007, when a study  on 12 obese women in a metabolic ward found that late-night eating did not affect weight loss. The women went through three 18-day periods in which a similar range of calories (~900-1,000) were consumed: During the first period, the calories were distributed across 5 meals; during the second period, calories were only consumed from 9 to 11 a.m.; and during the third period, calories were only consumed from 6 to 8 p.m. Changes in weight, body fat, and fat-free mass did not significantly differ across the three periods. This study provides some preliminary evidence: In a highly-controlled setting where all the meals are prepared for you, the times at which you eat aren’t that important for body weight. But that’s not enough, we need more evidence! Namely, evidence outside a short-term metabolic ward study.
Perhaps as expected, eating a similar number of calories, although at different times throughout the day, does not seem to have a major effect on weight gain.
A 2012 study  provided a much different angle, in both study design and results. It was a 6-month trial on 78 obese police officers, and actually ended up linking night-time eating with enhanced weight loss. Officers who were assigned to eat most of their daily carbs at dinner (dinner-carb eaters) lost more weight, inches off their waist, and body fat than officers who ate most of their carbs before dinner (day-carb eaters). Moreover, these dinner-carb eaters showed better improvements in insulin sensitivity, blood lipids, and inflammation. However, dinner-carb eaters did start out with a higher average BMI, weight, and body fat percentage than day-carb eaters, which could partly explain the greater improvements observed in this group.
The findings of the prior two studies somewhat conflict with those of two recent trials, both published in 2013, which suggested that eating earlier in the day may distinctly benefit weight loss. The first  involved 74 overweight and obese women with metabolic syndrome. They were randomized into one of two weight-loss groups: the breakfast group and the dinner group, that both ate an identical amount of calories (1,400 kcal) for 12 weeks and differed mainly by the time at which most calories for the day were consumed (either at breakfast or at dinner). The breakfast group lost more weight and inches off their waist, and experienced a significantly greater decrease in fasting glucose, insulin, and HOMA-IR than the dinner group (although fasting glucose, insulin, and ghrelin were reduced in both groups). Furthermore, mean triglyceride levels decreased by 33.6% in the breakfast group, while they increased by 14.6% in the dinner group.
The second trial was a 2-week crossover in healthy young adults of normal body weight. This study found that completely restricting food intake between 7 p.m. and 6 a.m. led to a moderate decrease (-239 kcal/day) in total daily caloric consumption, a reduction that could lead to considerable weight loss if sustained long-term.
A final study yielded some rather inconclusive results about the superiority of early-day vs. night-time eating, but still suggested that both styles were effective for losing weight. This 105-day, crossover trial  involved 10 overweight and obese adult women in a metabolic ward. The authors found that during the first crossover phase (first 6 weeks), participants who ate most of their daily food at night (p.m. eaters) lost more fat and less lean mass than those who ate most of their food in the morning (a.m. eaters). However, during the second cross-over phase (when participants switched groups), a.m. eaters lost more fat than p.m. eaters. This could be partly explained by the higher body fat percentage at the start of the study among participants who lost the most fat (those who were first in the p.m. group and then switched to the a.m. group), which usually makes fat loss easier.
Out of this group of studies, one found superior weight loss in night-time eaters, two found superior weight loss in early-day eaters, and two others either found no difference or had mixed results.
While the scales are tipped slightly in favor of daytime eating (in terms of the number of studies that found superior benefits), it's pretty hard to reach any definite conclusions about which style of eating is better for maintaining or losing weight. The various studies on meal timing (which extend beyond what we covered here) are extremely heterogeneous in who they studied, what diets were used, and a ton of other factors.
What we can say is this: Eating at night isn’t a factor that will inherently break your diet or compromise attempts to lose weight. On the other hand, if you already know that your mind in a tired state will drift toward raiding the ice cream or snack stash, it might be worth trying a restricted eating period, such as 12 p.m. to 8 p.m., or even a wider one like 8 a.m. to 8 p.m.
It’s also important to consider meal timing in combination with other factors, such as circadian rhythms. Eating late at night is linked to disrupted circadian rhythms  for some people, and appetite-related hormones also show daily fluctuations. 
While late-night eating isn’t inherently bad, the impact of meal timing is probably quite different in different people: Some people have a tendency to binge on tasty foods when they’re tired, while others don’t get hungry in the hours before bed. Circadian rhythm factors are another reason why some might want to stay away from eating too much late at night.
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