How can I turn off all sounds and LEDs apart from my Alarm? How can I deactivate all sounds and LEDs, except the alarm? When I'm sleeping, I don't want to be disturbed by anything except the alarm.
Priority Mode added in Android 5.0, allows you to 'snooze' alerts until you turn 'All' mode back on or for a set period of time. You can trigger it by touching either of the volume buttons.

In Android 5.1, you can set priority mode to be enabled until your next alarm, which means you can set it right before going to bed and when your alarm goes off it will be back to 'All' mode.

NB. This also applies to 'None' mode as well as 'Priority'

As for the LED's i don't think it is possible with stock android to turn them off just when in Priority mode. It might be possible using an app such as Lightflow Lite (Full) which allows you to configure the LED notifications for your device.