The worst case was when I felt dizzy on the way home and staggered into the first chippy I found for a chip butty loaded with salt and vinegar. The poor ladies thought I was going to die as I looked rough. I put this event down to me being dehydrated quite a bit...
So yes I also have experienced this, and the fatigue never seemed to be related to anything (except when hungover).
Things I did to try and break this fatigue which worked for me, on a 7 mile each way hilly commute (or now 23 total as the good weather means can include some offroad stuff):
- no big breaky before I leave home (maybe small amount of bran flakes), usually wait until I get to work
- load Ipod with some nice music depending on mood
- take water on board on way to work
- fruit when I get to work
- golden syrup porridge when I'm starting to feel hungry (about 11)
- fruit throughout the day
- try and drink 1.5 - 2 litres of water at work
- find time for some quick leg stretches
- pasta about 2-3 hours before I leave work
- keep something sweet in my pocket for way home (don't always eat it but it's there)
- think about route options for way home (possibly include something interesting such as good climb, ST, use wind for fast section, etc.)
- load Ipod for good music on way home
- water on way home
One day a week (or when I feel like it) I take the car like this morning and don't beat myself up about it. And straight away, as the poster above said, I'm thinking about the next time I pedal commute which is a boost.
The worst days for me are when I feeling knackered about 4 o'clock, not long before leaving work. But, I've found this doesn't happen as much when I stick to the above.