My weight at the gym was even lower this morning, 67.5kg. I lost some fat mass, but also a little muscle mass. My trainer was a little concerned. The first thing he asked me was how was my sleep. I half-heartedly suggested that the sneezing fit I had on Friday in-the-middle-of-the-night might be to blame. But actually I had no heart to tell him that I had stayed up late on Friday and Saturday night to watch TV. *Sheepish*
As I’ve mentioned before, that’s all it takes to lose muscle mass. Improper sleep, including insufficient or interrupted sleep. You can build all the muscle you want through training and eating right, but it only takes a night or two of improper sleep to lose it all.
I think ultimately we are built with very simple muscle systems. Use it or lose it. Sleep well or no swell (i.e. no swelling of biceps).