Ubuntu preventing Windows 7 host from sleeping

[ubfan]

I got Virtualbox Version 5.1.28 r117968 (Qt5.6.2), running in Windows 7 Ultimate. I installed Ubuntu 16.04 LTS as a guest in Vb. Works great but doesn't let Windows go to sleep after the sleep timeout. I can put Windows to sleep manually but Windows cannot go to sleep on its own when Ubuntu is running in Vb. Any clues on how to fix this? I like to save power and want Windows to go to sleep on its own if a user forgets to put it to sleep manually.