Tokyo to Sapporo by JR Pass overnight

I have bought JR pass ticket. And I would like to go to Sapporo from Tokyo.I plan to travel at night.Could you describe to me about sleeptrains by using JR pass ticket ,please?

P.S. I have searched in website trains timetable (hyper...) but I have not found it.

Before March 26, you can take the Shinkansen to Shin-Aomori, then a local train to Aomori and the Hamanasu to Sapporo. Last possibl departure from Tokyo 18:20, arrival in Sapporo 06:07. From March 26, it will no longer be possible as night trains to Hokkaido will be discontinued.

http://www.hyperdia.com/cgi/en/search.html?dep_node=TOKYO&arv_node=SAP...

http://www.japan-guide.com/e/e2356.html

So many thank you for the detail.It's very helpful.
I will travel on 13 Jan 2015.So I and my friends tried to find Hamanasu line but we didn't found it until now.Thank you very much.

Sincerely

Sorry to disturb you again.
I found the route that travel by night train like you have sent to me only in this year but in Jan there's not. Is there any changing about night train?
Thank you

As indicated at the bottom of the search result page, Hyperdia's information is presently valid until December 31. To obtain information for January, consult Hyperdia when it is updated, or some other source. That said, there should not be any major change.

Note also that due to preparation works for the Hokkaido Shinkansen, the Hamanasu will not operate on the following days:
12/31
1/1
2/14
2/21
3/22
3/23
3/24
3/25

By coincidence, JR East just posted the following English press release today:
http://www.jreast.co.jp/e/pdf/tsugarukouji_kokuchi.pdf

Thank you very much.