Sleeping at work is so common in Japan that there's a word for it in Japanese — Inemuri (居眠り).

To properly understand Japanese culture you need to look at Japanese ideas about sleep. In Japan, people get respect for giving their best (for pushing themselves to exhaustion). Therefore, showing how tired you are is a statement — it says you're a hard worker.

When people say goodbye to their coworkers in the evening they don't say "have a nice night" or "have a nice weekend". They say " otsukaresama deshita ". This can be literally translated as "you are tired sir". It's the nicest thing you can say to someone — that they are tired.

In this context it's easy to understand why people get away with sleeping at work. Coworkers assume the sleeping person must be working too hard.

The Rules

There are rules of sleeping at work (inemuri):

1. You must sit up and look engaged (despite the fact that you're asleep). It must appear that you could wake up at any moment and do something great.

2. It's easier to get away with if you're the boss. Sleeping at work is a sign of confidence — it shows you're indispensable to the company and can get away with it. Junior staff can also get away with it because no one notices them.

Personal Experiences

I worked (several years) at a Japanese company and can confirm that inemuri is a everyday occurrence.

The weirdest thing for me was when I had 1:1 meetings with my boss. He would ask me to explain something or give a presentation — he would always sleep through it. It was weird to give a presentation when I knew nobody was listening.

My only hope that my boss might notice my good work: I might appear in his dreams.