Agree it may be mid-week sleep (probably the most likely reason)
If you've eliminated sleep as a possibility (i.e. you really are getting 8+ hours of un-interrupted quality sleep, each and every night, and that doesn't mean "I can get by on 5 hours because I'm tough")
If you've eliminated that and still have a problem, consider your carb intake. With 2-a-day rides in the cold, your metabolism is basically turned on full blast all day and all night. You're burning tons of calories, and sure the weight loss is great (been there myself) but after a while you just need fuel.
So... IF you've eliminated sleep as a cause, try adding a bowl of pasta or your favorite complex-carb fule every day. Me, I used to add a regular side-order size bowl of rice to my lunch on days when I commuted, worked out about right. Another indicator is if you're constantly hungry but feel guilty because you don't want to get fat... just feed the beast some carbs for a few days and see how you feel.