Love the bed at Sunroute Plaza Hotel. Slept well despite few hours and did not experience backaches.
Location is a stone away from subway (Shinjuku), 24-hour convenience stores (Family mart and Lawson), Macdonald. Best of all, walking distance (about 10 mins) from Takashimaya!
The airport limousine also alights passengers in front of hotel, a plus if you have bulky luggage.