Sleeping in public

Is it safe / acceptable to sleep for example at rail station? Or are there other ways to spend night for free? Other than couchsurfing?

Is it safe / acceptable to sleep for example at rail station?

It is generally not acceptable. In many places it is illegal. Nevertheless, you might see some homeless people or cheap traveler doing it. I recommend not to do it and to properly stay overnight somewhere.

Or are there other ways to spend night for free?

Haneda Airport's International terminal is open 24 hours.

Stations in Japan usually close between about 1am and 5am or so, so it's not actually possible to stay overnight in any case.

Cheap options are internet/manga cafes, Haneda is possible as Uji said but you will still have to pay for the train fare out there.

there are some 24 hour restaurants...

Many years ago I arrived on the last train once with no accomodation booked and slept in the station with the vagrants. At ~12:30am I got taken away by the local police!

I will add it was obviously my fault for not prebooking accomodation in the town.

Once at the police station I was treated really well by the local constabulary, they provided me with a bed, food, coffee and we talked happily almost all night. They thought it was quite a novelty!

there are so many internet cafes, spas, restaurants etc. that allow you to sleep on the cheap...there is really no need to stay in the station and if you do you will find yourself sleeping with homeless people. Japan is one of the best places for emergency cheap accommodation when you miss the last train etc...no idea what I would do if I got caught out in a UK city...would have to pay for an expensive hotel I guess.

Thank you for all the answers!