I am currently having my PHD. Not the kind where you need to lease out your soul to a slave trader for 3 years, but a sickness short for the Post-Holiday Depression.
I got it every time I return to work from holidays. Usually the longer and better the holiday is, more severe the PHD would be. The depression usually begins when you start packing up for the return trip, and would sometimes last 1-2 weeks, before having myself buried in work again.
Symptom: Severe depression. Feeling sad. Sigh for no apparent reason. Lack of motivation at work, and easily feel tired. It is a magnitude more intense than Monday blues, and it can totally destroy your effectiveness at work.
Prevention: Not having a holiday. I've not felt so down for the last 2 years when I did not take a break. Or maybe not coming back to work - that will definitely stop the whole depression all together. However, both approaches might introduce other undesirable side effects.