systemd-suspend.service, systemd-hibernate.service, systemd-hybrid-sleep.service, systemd-suspend-then-hibernate.service, systemd-sleep — System sleep state logic
systemd-suspend.service
systemd-hibernate.service
systemd-hybrid-sleep.service
systemd-suspend-then-hibernate.service
/usr/lib/systemd/system-sleep
systemd-suspend.service is a system
    service that is pulled in by suspend.target
    and is responsible for the actual system suspend. Similarly,
    systemd-hibernate.service is pulled in by
    hibernate.target to execute the actual
    hibernation. Finally,
    systemd-hybrid-sleep.service is pulled in by
    hybrid-sleep.target to execute hybrid
    hibernation with system suspend and pulled in by
    suspend-then-hibernate.target to execute system suspend
    with a timeout that will activate hibernate later.
Immediately before entering system suspend and/or
    hibernation systemd-suspend.service (and the
    other mentioned units, respectively) will run all executables in
    /usr/lib/systemd/system-sleep/ and pass two
    arguments to them. The first argument will be
    "pre", the second either
    "suspend", "hibernate",
    "hybrid-sleep", or "suspend-then-hibernate"
    depending on the chosen action. An environment variable called "SYSTEMD_SLEEP_ACTION"
    will be set and contain the sleep action that is processing. This is primarily helpful for
    "suspend-then-hibernate" where the value of the variable will be "suspend", "hibernate",
    or "suspend-after-failed-hibernate" in cases where hibernation has failed.
    Immediately after leaving system suspend and/or hibernation the
    same executables are run, but the first argument is now
    "post". All executables in this directory are
    executed in parallel, and execution of the action is not continued
    until all executables have finished.
Note that scripts or binaries dropped in
    /usr/lib/systemd/system-sleep/ are intended
    for local use only and should be considered hacks. If applications
    want to react to system suspend/hibernation and resume,
    they should rather use the Inhibitor
    interface.
Note that systemd-suspend.service,
    systemd-hibernate.service, systemd-hybrid-sleep.service, and
    systemd-suspend-then-hibernate.service should never be executed directly. Instead,
    trigger system sleep with a command such as systemctl suspend or systemctl
    hibernate.
Internally, this service will echo a string like
    "mem" into /sys/power/state,
    to trigger the actual system suspend. What exactly is written
    where can be configured in the [Sleep] section
    of /etc/systemd/sleep.conf or a
    sleep.conf.d file. See
    systemd-sleep.conf(5).