Young adults who tend to stay up late and wake up later have, on average, higher body mass index, waist size, and waist-to-hip ratio than those with earlier sleep patterns, even when physical activity is accounted for. This suggests that their internal body clock may influence behaviors like eating at irregular times or eating less attentively, which could relate to higher body fat levels.

We analyzed the available evidence and found that young adults with an evening chronotype — those who naturally stay up late and wake up later — tend to have higher body mass index and larger waist circumference compared to those with earlier sleep patterns. This pattern held even when physical activity levels were taken into account [1]. What we’ve found so far suggests that this association may not be just about how much people move, but possibly about how their internal body clock influences daily habits. For example, people with later sleep schedules may eat meals at irregular times, snack more in the evening, or pay less attention to what they’re eating — all of which could contribute to changes in body fat distribution. We did not find any studies in our review that contradicted this pattern. It’s important to note that we did not measure whether these habits directly cause changes in weight or waist size, nor did we examine whether shifting sleep times would change body measurements. The evidence we’ve reviewed leans toward a link between evening preferences and higher BMI and waist size in young adults, but we can’t say why this connection exists or if it applies to everyone. Our current analysis shows this pattern consistently across the data we’ve seen, but we don’t yet know how strong or stable it is over time. More research would be needed to understand whether adjusting sleep habits might influence these outcomes. In everyday terms: if you’re someone who naturally stays up late, you might notice your eating habits shift toward later hours — and that could be something to pay attention to, not because late nights cause weight gain, but because timing and awareness around food may play a role.