Suspend command in Ubuntu 16.04

As per Debian's wiki, with systemd this is the new command:

systemctl suspend

NOTE: This command still requires root privileges if there is another user's session open. It also doesn't work in Cron. In both cases you you will need to create extra settings.

Suspend method is still available on D-Bus, if needed:

dbus-send \
  --system \
  --print-reply \
  --dest=org.freedesktop.login1 \
  /org/freedesktop/login1 \
  org.freedesktop.login1.Manager.Suspend \
  boolean:true

boolean:true is the user_interaction parameter:

The user_interaction boolean parameters can be used to control whether PolicyKit should interactively ask the user for authentication credentials if it needs to.

Source