I have been sharing my advice on this subject ever since I got this macbook. MAKE SURE your sleep light is PULSING before you stuff it in the bag. If you don't, and it decides to not sleep properly, you will pull it out three hours later, very hot, OFF, and with a totally dead battery. I have experienced this three times and have learned from it. I sleep my laptop at least once a day, some days five or six times. Of that, about one sleep per two weeks fails to sleep. I close the lid, the sleep light turns on really bright, and STAYS that way. After waiting 2 minutes it's either asleep (pulsing) or it's never going to go to sleep. In most cases I have to restart the computer to restore its ability to sleep at all, at which point I just shut it down instead of sleeping it.

Not sure what's causing it, but traditionally sleep problems have been software-related. I have read numerous posts regarding this problem so it's not all that uncommon. I don't know if it's as common with macbooks as it is with the pros. I have also seen mac pros fail to sleep in the same way but you don't have the battery and heat issues complicating things so a restart is not such an inconvenience. The stopper seems to occur before the sleepimage file is written, so it will not be able to safe wake following this event.

When it fails to sleep, the backlight turns off and the sleep light turns on to its brightest setting and never starts to pulse. The hard drive is not spun down, and the computer is fully awake and generating heat, although the backlight is off. With the lid closed now, and especially if put in a bag, it gets very warm very quickly. Unable to sleep due to lid closing means unable to sleep due to keyboard timeout also, so it stays completely awake, heating up, until a few hours later it has exhaused its battery and triggers an emergency sleep due to low battery. This also fails, and the laptop eventually powers off "uncleanly". When you remove the laptop from the bag, it's usually still very warm, shut off, and the battery has one rapidly flashing light indicating "totally dead". It won't respond to a keypress so you will have to press the power button to start it. It hung while it was attempting to write the safe sleep (sleepimage) file so it will not be able to safe-wake, and instead reboots normally when the power button is pressed. FSCK has to run since the shutdown was unclean, so it will take longer than the average time to restart while it's checking the disk. You will likely get a warning "you should select sleep or shutdown and not just shut me off" dialog when it gets to your desktop.

Always wait for the sleep light to pulse before bagging it.