Experiment: sleep for X amount of hours (X being the amount of sleep I'd get pre-illness), try to be moderately active (i.e. go for a 20 minute walk), read a book, talk on the phone briefly and then see how this down-scaled version of 'normal life' works out. Result: I'm not well. Head foggy, speech slightly slowed down and I need to search for words (and use spell-checker). Hands shaking if I'm not sitting down. Mouth dry. Was on the verge of collapsing during walk.
I'm going to counter this little experiment with jasmine tea, my jammies, some knitting and a flick through The Knitter's Handbook I found secondhand (just £1!) during my brief excursion..