Aemiliana hit the nail on the head.
I had to do some last minute cramming for an exam last night and if I'd have looked at it as 'half a module of cognitive' and 'half a module of social' I would never have got anything done but instead I said 'one social lecture, one cognitive lecture, another social lecture then tea, one cognitive lecture, one social lecture, one cognitive lecture, meet friend for a cigarette, last two lectures, watch TV then bed.'
If you have lots of exams or an exam that is on lots of topics, I always find it useful to jump between two things to keep my mind active and I remember a lot more doing it like that as well.
If the stress gets too much then do go and see your GP. I was getting so stressed that I couldn't sleep and then I was tired all day because I hadn't slept so he gave me some sleeping tablets to get me back into a normal sleep rhythm and I was right as rain again in less than a week.