A know EXACTLY how you feel. Personally, I am a very active person so I work out a lot, but ever since I found out I have IBS (12 months ago) I can't get my butt out of bed and I HATE it!!! I have to get up for work at 5:30 am so on the weekends I used to sleep in until 6:00 but in the last twelve months I can't get out of bed on the weekend and it is driving me crazy!! Whenever I have an attack of IBS I can sleep for days and days at a time until my system regulates itself. I think a side effect of any type of gastrointestinal difficulty is tiredness.
Several years ago, I kept falling asleep early and waking up late so I went to a dietician and we found out that I wasn't eating enough calories. Are you getting enough food?? You need carbohydrates for energy. Thank goodness Heather's diet is full of complex-carbs so that should help, but make sure you're getting enough.
I also highly recommend any type of exercise. It has been very cold here too, which doesn't help the motivation, but we're having a chinook today!!!