[rory]

There is a bunch of experimental evidence now that animals replay some learned tasks in their mind during sleep. For example:

Louie, K. & Wilson, M. A. Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep. Neuron 29, 145-156 (2001).

There is also evidence that if the replay is disrupted via stimulation during sleep, the animals do not retain the learned task as well:

Wilson, M. A. Hippocampal memory formation, plasticity, and the role of sleep. Neurobiology of Learning and Memory 78, 565-569 (2002).

So instead of thinking of your dreams as being bad, think of it as your brain rehearsing what it has learned so that you will remember it better. It's good to dream about what you are working on or learning.