Logs of Ubuntu going into Sleep and waking up?

In case your Ubuntu is using systemd, you can use:

journalctl | grep systemd-sleep

You should get something like:

Jul 31 16:02:51 johndoe systemd-sleep[5958]: Suspending system...
Aug 01 15:34:03 johndoe systemd-sleep[25509]: Suspending system...
Aug 02 09:01:40 johndoe systemd-sleep[25509]: System resumed.
Aug 02 16:59:16 johndoe systemd-sleep[23496]: Suspending system...
Aug 06 08:46:04 johndoe systemd-sleep[23496]: System resumed.
Aug 06 17:05:08 johndoe systemd-sleep[6071]: Suspending system...
Aug 07 08:41:33 johndoe systemd-sleep[6071]: System resumed.
Aug 07 16:48:01 johndoe systemd-sleep[14495]: Suspending system...
Aug 08 09:06:48 johndoe systemd-sleep[14495]: System resumed.
Aug 08 16:38:55 johndoe systemd-sleep[14729]: Suspending system...
Aug 09 07:53:48 johndoe systemd-sleep[14729]: System resumed.

The favorite place for me (but not the only place) is accessed using:

gedit /var/log/syslog

To view older log records look at:

gedit /var/log/syslog.1