Merely turning your phone on may be enough to prevent it from sleeping. What I'm about to tell you is 100% repeatable. I've done it about ten times now; it has happened every time. I'm running the stock Android 2.1 ROM from Sprint.

<Background> When Android boots, it preloads a huge number of apps into RAM. Essentially, it's starting JVM processes and loading the classes of applications without actually executing any of their code. Android does this to reduce the lag you would feel when activating one of these apps. These preloaded apps are said to be EMPTY. There's no real penalty for preloading these apps since they're loaded in the background in otherwise idle CPU cycles and their processes will be terminated as needed to free up RAM for applications you begin using. </Background>

Although it should be impossible, one of those preloaded apps is preventing the Hero from sleeping. By process of elimination, I have found which one it is: Footprints.

Here's my test case, which I can repeat every time:

1. Boot the phone from cold.
2. Go have a beer. I'm only partially kidding: it doesn't have to be beer, but do something (not on the phone) for a few minutes. The phone needs to be allowed to finish preloading all of its applications, which it will continue to do in the background even after the launcher becomes usable.
3. Return to your phone to find its screen off. You only think it's sleeping now. It's not. The CPU is quietly draining your battery.
4. Unlock the screen and press "Menu, Settings, About phone, Battery."
5. Notice that the "Up time" and "Awake time" are the same. D'oh!
6. Go back Home and fire up a task manager. I use Estrongs.
7. Find and kill the Footprints app, which should be listed as "EMPTY."
8. Now press the power button or simply wait for the screen to turn off.
9. Wait for 10 seconds.
10. Press the power button again to turn the screen back on.
11. Unlock the screen, go Home, and again press "Menu, Settings, About phone, Battery."
12. "Up time" will be approximately 10 seconds longer than "Awake time."

Conclusion: Killing the EMPTY Footprints app allowed the device to sleep.

You don't have to exactly follow the above sequence of steps, either. I've done other things after booting but before killing Footprints, including making phone calls both with the built-in phone and with Sipdroid, killing other applications besides Footprints, turning the screen off and on, and rechecking the Battery info. Basically, until you've started enough other apps that the Footprints app gets killed to reclaim memory, your device will not sleep.

 

It seems killing Footprints isn't always enough to make the device sleep properly. It works more often than not, but it's not guaranteed. And when it doesn't work, I can kill every app on the phone, and still be stuck awake. When that happens, I have to power cycle the phone. When it comes back, I have to go kill Footprints, and then usually it'll sleep. There is something very fishy here. I guess I'll just learn to live with the phone not sleeping. As long as I can get a day's worth of runtime out of the battery, I'm not too upset. Still, the phone can get pretty warm when it's burning CPU cycles rather than sleeping as it should.

 

It's not a "task killer," per se. It doesn't automatically kill tasks. And I don't indiscriminately kill tasks manually either. I understand Android's system of preloading processes, and I'm fine with it, except that it seems that even an EMPTY process can still hold a partial wake lock, and that's messed up.

Also, I've had a legitimate use (other than wake/sleep issues) for killing a process. The Facebook app sometimes gets into a failure mode where the server returns a "null/-1" response, and then the Back button will no longer take the app back to its main menu. Switching to the Android home screen and then invoking Facebook again just takes me back to the busted screen in Facebook. The only way to make Facebook work again is to kill its process and start it over. For that, you need a task manager.

 

Eh, I'm upgrading to the EVO 4G as soon as possible anyway, so I'm not too concerned, as long as it's not also a problem with the EVO. I only got the Hero to tide me over because Sprint decided to shut down the Helio (postpaid) division of Virgin Mobile ten days before releasing the EVO. Bad timing. But thankfully Sprint has that 30-day guarantee, so I can switch phones (though I may have to pay a $35 restocking fee).