I've been on a...well, not a diet, but weight-loss thingy (call it a diet and I will fail ) over the winter. Out here, it gets colder, from the 100s down to the 60s and 70s during the winter and it is great riding weather with very few rainy days. For the first time, I wore a HRM through the winter rides. Well, my calorie counts on the same loop went down with the temperature. Couldn't figure why till I realized that my body has to work harder in the heat to try and cool myself during the rides. I was riding around dragging two full waterbotles because I was not under the extra heat stress, and burning less calories.

Now if you are indoors, in a nice cool room on a trainer, and not outside in the sun and dealing with the warmth too, that might be a factor. You are also not dealing with things like headwind and hills. I have found my Heart rate monitor to be invaluable in determining just what my body is really doing. Oh, and I use only daylight balanced bulbs in the house; can't quote any scientific evidence but the light is much less stressful on my eyes than those orange tinted bulbs.