systemd supports four general power-saving modes:
Settings in these files determine what strings will be written to /sys/power/disk and /sys/power/state by systemd-sleep(8) when systemd(1) attempts to suspend or hibernate the machine. See systemd.syntax(5) for a general description of the syntax.
The default configuration is defined during compilation, so a configuration file is only needed when it is necessary to deviate from those defaults. By default, the configuration file in /etc/systemd/ contains commented out entries showing the defaults as a guide to the administrator. This file can be edited to create local overrides.
When packages need to customize the configuration, they can install configuration snippets in /usr/lib/systemd/*.conf.d/ or /usr/local/lib/systemd/*.conf.d/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. The main configuration file is read before any of the configuration directories, and has the lowest precedence; entries in a file in any configuration directory override entries in the single configuration file. Files in the *.conf.d/ configuration subdirectories are sorted by their filename in lexicographic order, regardless of which of the subdirectories they reside in. When multiple files specify the same option, for options which accept just a single value, the entry in the file with the lexicographically latest name takes precedence. For options which accept a list of values, entries are collected as they occur in files sorted lexicographically. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering of the files.
To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to /dev/null in the configuration directory in /etc/, with the same filename as the vendor configuration file.
The following options can be configured in the "[Sleep]" section of /etc/systemd/sleep.conf or a sleep.conf.d file:
AllowSuspend=, AllowHibernation=, AllowSuspendThenHibernate=, AllowHybridSleep=
If AllowHibernation=no or AllowSuspend=no is used, this implies AllowSuspendThenHibernate=no and AllowHybridSleep=no, since those methods use both suspend and hibernation internally. AllowSuspendThenHibernate=yes and AllowHybridSleep=yes can be used to override and enable those specific modes.
SuspendMode=, HibernateMode=, HybridSleepMode=
SuspendState=, HibernateState=, HybridSleepState=
Example: to exploit the "freeze" mode added in Linux 3.9, one can use systemctl suspend with