If I couldn't sleep in a comfortable temperature, I'd choose too cold over too hot. Even if its uncomfortably cold, I can sleep fairly well, and I think sleeping well is the main key to dreaming. Having slept in a room that gets possibly into the 40's during winter, and over 90 in the summer, I find colder to be preferable. Plus, I can get that nice cozy feeling under the covers, which is very relaxing, especially in the early morning, prime time for dreaming.