Speaking from personal experience, other major events which can result in a Japanese salaryman sleeping at their office include "the day is Monday", etc. Apparently moving one's toothbrush is a sign of commitment in romantic comedies? I had a toothbrush and shaving kit in my desk for three years. My boss had three changes of clothes.
This is very off-topic, but I am curious about this cultural habit of Japan. Does spending 2X more hours at the office actually get 2X more work done? I tend to lose focus rapidly after 9 or 10 hours.
searchyc.com + [patio11 salaryman] for anecdotes, but the short answer is "No, it is absolutely not the case that the hours make Japan more productive."
