One of the best ways to ensure that your exam revision is effective, efficient and manageable, both mentally and physically, is to take regular breaks - no matter how much work you think you need to get done!
The simple fact is that your brain is not able to work for a long time without a rest. If you've ever tried working for a whole day without a break, you'll know that doing so actually leads to massively unproductive results. Even if you don't grow tired and lose your concentration - which is the most likely outcome - you'll certainly start suffering from cabin fever!
At the same time, you don't want to go the other way and start taking too many breaks. If you spend more than 50 per cent of your day walking in the park or watching TV instead of revising for your GCSEs or A-levels, you're doing something wrong.
Conventional wisdom says that you should spend no longer than half an hour in revision before having a break. Breaks should last between five and ten minutes - no longer, otherwise you'll get out of your work rhythm. Try going outside for a short walk if you can, as the air will clear your head.
Do take a longer break for lunch and also set a strict cut-off point for the end of your working day. Having the time constraints put on you by breaks and cut-off points helps you to work more efficiently.