To answer M Fabien, the 8300HD attempts to enter sleep mode several times between 1-5 am. I find this really annoying since I watch a number of programs between midnite and 4 am. If I miss the 5 minute warning (during a snack break, or if I get distracted for a few minutes), the unit will enter power-save mode. When this happens, the hard drive powers down, and the video buffer gets flushed. Occasionally, during power-save mode, the unit even changes to another channel by itself! Then when I return and wake up the PVR, I cannot rewind and watch what I missed.
This will not entirely satisfy you but if I were you, I would record each program that I view between those hours, even if I view them "Live". First it calls for an activity which the DVR cannot ignore then if and when the DVR stops operations, you can power up the unit, rewind and playback the recording from the place left off.
Or you can from time to time press the "Pause" button which forces reaction from the buffer and again this will tell the unit that there is activity.
Hope this helps.
Samsung LN52B750, SVS PB12-ISD, Harmony One, HD-A1, BDP-1400, Oppo DV-970HD, Yamaha RX-V371