Disregarding GUI you can always put your Linux system to sleep from command line. This is especially useful if you GUI breaks from some reason and becomes unresponsive to suspend command or you wish to suspend your system remotely. Suspend also known as sleep is a state where your system current running state is saved directly to RAM and remains dormant until awaken by some sort of wake up event such as pressing power button, opening laptop lid or mouse move.

Since RAM is a volatile memory and requires electrical power to hold data your system while in suspended state continues to use electrical power. To suspend/sleep your Redhat/Fedora system you can enter the following command:

rhel7~]$ systemctl suspend

Depending on the system settings you may be required to execute the above command as a root user. The system will not go into a suspended state while users are logged in. You can overwrite the suspend inhibitors with -i option.

rhel7~]$ systemctl suspend -i

Just to be complete the suspend state is not to be mistaken with hibernate state. In hibernate state the system running state is save to hard drive thus not requiring electrical power to maintain hibernation.