Research has shown an association between shift work and metabolic health. Specifically, shift workers are prone to obesity and type 2 diabetes. However, there is less research data about a variable sleep schedule that is not related to shift work. I'm referring to individuals who have one bedtime during the work week but a later bedtime on the weekends. This study researched the effects of going to bed later on weekends and obesity as well as metabolic health. They studied all women with a mean age of 52 years. The researchers collected information annually on sleep times and duration with a sleep diary for about 5 years. They recorded the patient's weight and drew blood to check for insulin resistance (a marker for pre-diabetes). The results showed that a greater variability in bedtime and going to bed early was associated with an elevated body mass index. Interestingly, the mean bedtime and going to bed later were not related to body mass index. However, variability in bedtime and going to bed later on the weekends was associated with an increased rate of insulin resistance, even after adjusting for factors including sleep duration.
The authors speculate on the reason why later weekend bedtimes could contribute to insulin resistance. One possibility is that there is more exposure to light at night which could alter melatonin and possibly increased consumption of food. However there was no significant increase in weight over time despite later bedtime. It may be that people who go to bed later on weekends are eating more simple carbs but don't necessarily gain weight. Increased carb intake could be contributing to the insulin resistance. More studies will need to be done to see how clinically important these effects are, as the statistical differences were real, but small.