Depending on the sleep mode, usb wake on may be enabled (Tv tuner's, or backup hard drives). Also programs with schedules such as virus scans, windows update, and backup programs can indeed wake your computer up from sleep depending on what level of sleep it is. Bad programming for the BIOS can also make it wake up after so long (old computers) for no apparent reason.
Try going into hibernation mode, it's not as fast as sleep s1/s2 but it certainly uses less power then S1/S2 (S3=hybrid and is almost the same as hibernation except your ram/motherboard remain on, in hibernation everything turns off and your ram is saved on your hard drive for quick boot.)
Hibernation cannot be awaken from without pressing the power button, saves energy since your PC is turned off, and is much faster then a cold boot. Did I mention that when you boot from hibernation as soon as windows loads it's ready to use since all the programs were loaded already since you last hibernated.