Why does my computer keep waking from sleep on its own?

Something is issuing a wake request, and both major operating systems can tell you what. Rather than guessing, query the system for the most recent wake source; the answer is usually specific enough to fix immediately.

The common sources fall into a few groups.

• Input devices. A mouse on a desk that receives the slightest vibration will wake a machine repeatedly, and this is the single most frequent cause. Each device can be individually prevented from waking the system in its properties.
• Network activity. Wake-on-network features exist for remote management and are enabled by default on many machines. On a busy network, broadcast traffic can trigger them. Disabling the feature on the network adapter resolves it.
• Scheduled tasks. Maintenance windows, backup software and update services are permitted to wake the machine to do their work. These are configurable, and the scheduler will show which tasks hold wake permission.
• Connected peripherals. Some devices, particularly those with their own power, signal the host when their state changes, which can wake the machine when they power on or off with the room lights.

There is one case that is not a fault at all. Modern standby modes keep the machine partially active by design so it can receive messages and updates, similar to a phone. A machine using that mode is not sleeping in the traditional sense, and screen activity or fan noise is expected behaviour rather than a wake event.

If the wake source reports something generic and no setting helps, disabling wake timers entirely in the power configuration is a blunt but effective final step.