Why does my phone suddenly come ot life in sleep mode?

[AC Question]

Have seen my phone suddenly come to life (home locked screen) a few times recently when in black screen/sleep mode - both when pugged in charging and when sitting unplugged. No text, no call, no battery alert, no email/facebook alert etc. Why is it doing this?

 

[B. Diddy]

Welcome to Android Central! Which phone?