I think who is going to Japan may like this informations about "DVD and relaxation shops" where it's possible to sleep, relax, and jerk off.
Recently few english websites wrote about this kind of places, but my first time there it was in 2008. This shops are very good places when you need to take a break a couple of hours or you want to stay overnight for a very cheap price.
Before you read this I have to say that girls can go to this shops (but not always, read below) but it's forbidden to share the same room, even for some minutes, and they don't provide girls, so it's mainly to relax/sleep/jerk off with DVD, not for sex or sex-related things.

The system
When you enter in the shop near the entrance there are some small baskets, you have to take one, and then start to go around the shop where there are hundreds of DVDs. They have mainly adult DVDs, in different sections. Adult DVDs in Japan are censored, as probably you know, that's why in my opinion the best are "gravure DVD" because they are not censored (you can't see genital area but this videos are usually very exciting), or DVDs with strong scenes (bukkake, etc.), so even if they are censored you can enjoy at watching them.
They have standard movies too, even in english, and some japanese magazines and manga.
You can take up to 6 different DVDs. It's important you don't take the whole box, but just the small box with CD that is inside each DVD box. I know it's not clear, but basically you have to keep there the cover. (Don't worry, even later you can change your DVDs anytime, for free, just pick up new DVDs and go to cashier to ask to change.)
Then you have to move to the vending machine for the ticket to "book the room" or to buy other services. It's completely in japanese language, but it's very easy. If you can't use it you can ask help to the shop assistant. With this vending machine you buy a ticket that you have to bring to the counter, and they will assign you a room. They will ask you which kind of room you prefer (read below), if you smoke, and if you need something more.
Foreigners are welcome, and they even have a paper in english to explain you the system, so don't worry at all.
With the key for the room they will give you a very cheap "jerk off toy" (but you can buy better ones) and some condoms. Condoms are not lubrificated and not for sex, but you can wear them to don't make you dirty during jerk off.
Then you can go to the very small room, close it, watch DVDs and enjoy or take rest.

How to find this shops
In Tokyo the main chain is "Takarajima 24" that is everywhere, but not in the other cities (they just have 2 shops in Osaka). Many districts in Tokyo have 5 or even 8 different Takarajima 24. It's impossible don't find them if you go around Tokyo, but if you want you can search on their website to find one. I didn't count how many shops they have in Tokyo, but I guess maybe 100 or 200!! In Takarajima24 girls are welcome, but you can't share the same room.
In other cities "Kintaro DVD" is the most popular, and girls are not welcome at all.

How are the rooms
Rooms are tiny, but cleaned. I have attached some photos I downloaded from their website, so you can see all different rooms. Usually they have available just chair room or flat room, it's very difficult to have "loft room" that is bigger than others.
Rooms offer 100% privacy. I say this because in standard net-cafe in Japan it's quite difficult to relax because rooms don't offer very good privacy.

Price
Remember: 1000yen are about 75HK$, or almost 10US$.
It's not possible to book the room. You can keep the room and go out from the shop (for example to have dinner), but I mean it's not an hotel.
Prices are usually: 1000yen for 2 hours, 1500yen for 3 hours, 2000yen for 6 hours, 3000yen for 12 hours. In the 12 hours plan they usually borrow you a bath towel,  but it's not the same in all shops. For each 30 minutes more you have to pay 500yen.
It means that you can sleep one night in the best parts of Tokyo for just 225HK$.
In few shops you can even buy a monthly card, and you can save some money, if you go sleep there everyday.

Adult toys
To have a better jerk-off experience you can buy a sex toy like Tenga. They have about 20 different toys, and they cost from 200 to 2000 yen. I recommend to choose randomly from 1000yen list. They have the famous "Tenga" too, but I prefer other ones.

Other Services
Usually they have cheap food, drinks, a microwave, but I think it's better to eat somewhere else. They can rent a blanket for 200yen, or a bath towel for 100yen, so you can take a shower too. If you really need something it's better you check on internet before go there, I mean that not all shops are same. It's possible to use internet in the room, but in Tokyo they need a copy of your passport too.
As I already wrote, they don't provide girls.

Hygien, gay, safety
Rooms are cleaned, I never had bad experiences. I read on internet that in few shops, for example in Shinjuku 2-chome or in "Ikebukuro Van"  few men meet to jerk off together in same room or suck each other, wich is illegal for the rules of the shop. I always went to other shops.
Safety is very high, and you will not have any problem, don't worry.
Even if rooms are cleaned, I usually prefer to rent a blanket or towel, and stay on it.

My favourites Takarajima24 in Tokyo
Of course I didn't try all Takarajima24, but I think I tried more than 30 and from my experience I can say that some are better than others.
I put here links of shops that I prefer, in each link you can find the map and the address (in japanese):
- http://takarajima24.com/shop/t_akihabara.html : in the Akihabara district there are 3, all are good.
- http://takarajima24.com/shop/t24_ueno.html : big shop in Ueno, rooms have double monitor
- http://takarajima24.com/shop/p_kabuki.html  : in red light district "Kabuki-cho" in Shinjuku. Very big shop but on the main street, so it can be embarassing to go inside for some people.
- http://takarajima24.com/shop/t24_shinjyukunishi.html : another in Shinjuku (but this one is in the West part), its' quite big.
- http://takarajima24.com/shop/t24_ookubo.html  : this in Okubo is quite big.

I think it's important to choose bigger shops because they have more DVDs to choose from.
And especially I recommend all shops that offer Sound Proof rooms: http://takarajima24.com/sound_proof.html  It's japanese page but you can use Google Translate, it's very easy to understand where they are. If the room is not Sound Proof you can hear some noise from the corridor, japanese people are usually quite, but if you want to sleep with silence you have to get a Sound Proof room.
In Sound Proof rooms you can keep the TV audio switched on, in other rooms they will provide you headphones.

Photos
I attach some photos of the shops. I took them from their website (I hope it's ok for the forum rules). Sorry but I never took photos there, and I think it's forbidden to take photos of from-desk and DVDs.

Any questions?
I hope you liked my small report/guide, if any question feel free to ask!