Startup chime while sleeping

[Bambina]

I have a MacBook Pro 13" mid 2012. After leaving my MacBook sleeping for a long period of time (1h-2h) with the lid down I sometimes hear the startup chime. Once I was able to immediately check what was going on and I saw that the screen was like coming out of hibernation (semi transparent white screen and loading bar)

Any idea why this happens? What can I do about it?

Thanks in advance

 

[Fishrrman]

Something is causing it to reboot while the lid is closed.
Not sure what...

 

[fromsixtozero]

Same thing with my Macbook Pro 15 late 2011. It is in sleep mode and suddenly it chimes in the middle of the night. I would like to know what causes it too....

 

[Weaselboy]

Same thing with my Macbook Pro 15 late 2011. It is in sleep mode and suddenly it chimes in the middle of the night. I would like to know what causes it too....

Do you happen to have any other devices like an iPhone or iPad connected to the USB ports n the MacBook? I noticed sometimes if I do that when the iPad starts charging the MacBook will make that chime noise like when you attached power.