I've always attributed gain weight during employed periods to the fact that I compensate my lack of sleep or shortened sleeping pattern with greater food intake. During unemployed periods, I eat less.
Apparently there's more to it:
We've known for some time that sleep deprivation is associated with weight gain and obesity in the general population, but this study shows that getting enough sleep — even just two hours more — may be as important as a healthy diet and exercise for new mothers to return to their pre-pregnancy weight
That reminds me, I need to sleep.