Well, to get your body cycle back on track, try to go to sleep at a constant regular time each night and to wake up at about the same time each morning; break the bad habit and form the new and good. If you're really tired and want to sleep then you could try having a hot chocolate before you go to bed because that will make you feel all warm, more relaxed and sleepy. Taking a hot water bottle to bed too for comfort and to make you feel more safe as well as for warmth sake may also become of use to you.
Where your diet is concerned, make sure you're getting your 100% daily quantity of iron because otherwise a lack of this leads to anaemia, where feeling sleepy, exhausted and drained are some of the symptoms.
Try not to multi-task doing your revision at the same time as TSR. Don't procrastinate and your time working will be more efficient.
The bad dreams nay just be because of exams coming up. Because they're coming up, you haven't really got to time to let your feelings out in a diary to release your emotions. But what you can do is take breaks in between revision and reward yourself with watching a programmme or talking to friends etc.