I had a mild case of mono about this same time last year. They tell you not to play hockey not only because exercise will make you way tired, but because your spleen and liver can swell. Taking a blow to the body (such as a puck) with a swollen spleen can equal spleen rupture, which can equal death. I don't know if it's always obvious, but in my case I could definitely tell something in my guts was swollen.
Even though my case was pretty mild, I could really tell the difference of how I felt when I got enough sleep versus when I didn't.
On the positive side, there's some kind of ongoing study out in Salt Lake (about 45 minutes from where I go to school) that gives you $200 a pop for your mono-laced plasma twice a week. I made $1200 and used the money to buy new pads to reward myself for being good and not playing.
Easiest/most lucrative job I ever had...
I don't know if there are similar programs elsewhere but if you like I'll PM you the name and number of the guy who runs it in Salt Lake and maybe he can tell you.
Feel better.
~Brent