In class, in meetings, and in trains (fairly often on strangers’ shoulders) and for most of a daytime bus or plane ride, it seems nodding off is the most natural of all Japanese reactions.
According to a recent edition of the BBC radio programme The Why Factor, cultures can be divided into three by how they sleep, with the two who don’t keep it all for the night being divided into siesta cultures (like Spain) and nap cultures (like, as they mentioned, Japan).
I think there are also both more random and deeper reasons.
As an example of the former, the main reason that napping in class is okay in Japan is that schools rarely stream by level and so there are students in the lesson for whom the teacher’s best possible hopes is that they sleep quietly rather than read manga, play with their mobile phones or get actually disruptive. Parents usually don’t mind either, because the useful study is done in cram schools in the evening. And when it comes to university, it’s almost impossible to fail and club activities are seen as incredibly important, so nodding off is only natural.
In a similar way, Japanese companies often tell people to come to meetings for no reason at all, and sleeping is considered a rational way of dealing with that – as well as a healthy sign that you are working yourself to death as you should.
For reasons like this, people simply get into a habit of sleeping, and that habit sticks. I do believe there are deeper reasons, though.
The main one is that there is no pressure to have internal motivation to keep yourself going, so if the social pressure is off, why not sleep? More specifically, that ridiculous pressure in the UK (or just London?) to rush around in your free time to tell everyone on Monday how cool and productive your weekend was doesn’t really exist – hence the “How was your weekend?” “I slept and cleaned my room” conversations that are the bane of Eikaiwa teachers.