Two PCs waking up by themselves at night

Question

My first post, found SU looking for an explanation to the odd behaviour of my PCs.

Last night I was back late and used my 2 PCs (details below) before going to sleep at 3.20-ish in the morning. I always put them in 'hibernation' rather than fully switching them off.

At 3.30-ish the desktop woke up by itself. I got up and logged in to check what switched it on, couldn't figure it out as no unusual process was active. I turned it off (hibernated) again, but one minute later it woke up again. Was too sleepy to look into it and simply unplugged it from the socket (I know, bad, but at that time I was too sleepy to deal with it). This would be bad enough but...

About 5 minutes later my OTHER pc woke up too! Again I got up, logged in and looked for suspicious processes/programs, couldn't notice anything. I turned it off by fully switching it off this time. If it woke up again I don't know, I then managed to fall asleep and I wasn't woken up any more.

I can't be 100% sure it never happened before, after all at that time I'm usually asleep and wouldn't wake up, last night was an exception as I had just gone to bed. Though I have reason to believe it was the first time it happened.

Details:

PC1: Lenovo desktop Win8.1, Win10 'reserved', about 20 months old.

PC2: Dell laptop Win7, Win10 NOT reserved, about 3 years old

Both PCs connect to internet via Wi-Fi (no ethernet cable), have Kaspersky ISS, Malwarebytes and Superantispyware installed. Yesterday I had run a session of software, drivers (NVidia) and plugins updates and scans on both computers so everything was fully up to date and no malicious software was found.

Any idea before I call the exorcist?

Thank you in advance

EDIT1: Windows Media Center not installed on either PC, and Windows Update not 'allowed' to wake up the PC