Why does my computer wake itself up or refuse to go to sleep?

Something is asking it to wake — a scheduled task, a network signal, a peripheral or a driver. The system will tell you what, if you ask it the right question.

Difficulty: intermediate
Time: 30 min
Read: 4 min
Safety: caution

Short answer

Both platforms record what woke the machine. On Windows, a command-line query reports the last wake source and which devices are permitted to wake it; on a Mac, the system log records a wake reason. The usual culprits are scheduled maintenance and updates, a mouse or keyboard on a desk that gets knocked, wake-on-network settings, and a connected device sending a signal. Disable waking for the device responsible rather than disabling sleep altogether.

Do this first

What is the difference between sleep, hibernate and shut down?

Safety

A laptop that wakes inside a closed bag can get very hot with no ventilation, which is bad for the battery and, in rare cases, a genuine hazard. If a machine repeatedly wakes in a bag, hibernate or shut it down before packing rather than relying on sleep.

Step by step

Find out what woke it, rather than guessing.Windows records the last wake source and can list every device permitted to wake the machine, both through its power configuration command-line tool. macOS records a wake reason in the system log. This turns the problem from a mystery into a named device or task.
Check the obvious physical causes first.A mouse on a desk that someone brushes, a wireless mouse on a shared surface, a games controller, or a laptop lid that is not quite closed. These account for a large share of cases and are solved by moving something.
Look at scheduled maintenance.Both platforms run maintenance, updates and backups at set times and will wake a machine to do it. That is intended behaviour and can be turned off in the scheduled task or the update settings, at the cost of that work not happening.
Check network wake settings.Wake-on-LAN and its wireless equivalent let another device on the network wake the machine. It is useful for remote access and a nuisance otherwise, and it can be triggered by ordinary network chatter on some adapters. It is a setting on the network adapter.
Deal with peripherals individually.Each device that can wake the machine can usually have that permission removed in its properties. Remove it for the ones you do not want — typically the mouse and network adapter — and keep it for the keyboard if you wake the machine that way.
Now take the opposite problem: it will not sleep.Something is holding it awake. Windows can list active power requests, and macOS shows processes preventing sleep. Media playback, an open video call, a download, a file being written over the network, and a disc in a drive are the usual holds.
Check for a stuck application.A media player that thinks it is still playing, a browser tab with audio or video, or a presentation application will legitimately assert that the display must stay on. Closing it releases the hold immediately.
Consider the firmware and drivers.Sleep behaviour is where firmware and driver bugs surface most visibly. If the machine has never slept properly, check for a firmware update from the manufacturer and update the graphics and network drivers before spending longer on settings.

If it doesn't work

Wakes at the same time every night

Cause: A scheduled maintenance or update task with permission to wake the machine — Fix: Find the task in the scheduler and remove its wake permission, or change its time. Do not disable updates themselves.

Wakes seconds after being put to sleep

Cause: A device asserting a wake immediately — commonly the network adapter or a USB device — Fix: Query the last wake source, then remove that device's wake permission. A USB device that does it repeatedly may need a driver update or a different port.

Laptop is hot in the bag with the lid closed

Cause: Woke after being packed and could not sleep again, or lid-close is set to do nothing — Fix: Set lid-close to sleep or hibernate, and hibernate before travelling. Check the machine is not on a modern standby setting that keeps it partly awake.

Screen will not turn off during video

Cause: Correct behaviour — the player is asserting a display request — Fix: Nothing to fix. If a player continues to hold it after being closed, quit it properly rather than closing the window.

Mac wakes for network access

Cause: Wake for network access is enabled, allowing sharing and remote access to wake the machine — Fix: Turn it off in energy settings if you do not use remote access or shared printing to that machine.

Questions people ask

Is waking for updates a bad thing?

No — it is how a machine that is off in the evening stays patched. The problem is usually that it wakes somewhere it should not, like a bag. Fix the location, not the updates.

Should I just turn sleep off?

It removes the symptom and costs you battery life and unnecessary heat. Find the wake source; it usually takes ten minutes.

Why does my desktop wake when I switch a light on?

Almost always a mouse on the same desk being nudged, or a shared surface transmitting vibration. Occasionally a power supply reacting to a mains disturbance. Try removing the mouse's wake permission first.