I had this happen frequently. To solve it immediately rotate the alarm take down and push the battery compartment, it'll pop open, remove the battery. You may have to do this to all until you find the culprit. Chances are there's dust, battery dead or a tiny bug can set these damn things off. Now, how to make sure it doesn't happen again? If your alarm has an i or I in the model number get rid of them. They're detecting ions, if you live near the beach like I do, just the waves crashing will set them off. Great huh? These are old technology cheap units apparently. Get upgraded with photoelectric units. Once I did this it no longer woke me out of a sound sleep.
Here is a link that might be useful: If You Have These Smoke Alarms, Get Rid Of Them!
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