Now, many things can cause this: mostly hardware interrupt handlers containing instructions to wake up. I first Googled this problem with the phrase: macbook does not stay asleep, but the Apple discussion board is completely disorganized. At the top of the page, it announces "1 solution" but doesn't give you a hint as to where it may be, in pages of discussion! I found it eventually, but I thought it would be helpful just to say:
... turn off Bluetooth.
It's in Systems Preferences under Hardware -> Bluetooth. Your computer will then get the rest it needs, without pills.
Addendum:
If the Bluetooth hardware is failing, the Bluetooth symbol has a little squiggle through it.
This makes the system write a hardware error to the system log (/var/log/system.log) every second. This wakes your Macbook consistently.
In this case, without repairing the hardware, you'll need to put your machine into hibernation, instead of sleep. There's a useful widget for this, called Deep Sleep.
Your Bluetooth hardware may start working again. Anytime you see the grayed out Bluetooth symbol in the upper right, without a squiggle, you'll be able to sleep normally ... if you turn Bluetooth off.
