What Does This Error Mean?

MacBook not sleeping is most often caused by an app holding a sleep prevention assertion — open Activity Monitor, click the Energy tab, and look for any app listed under Preventing Sleep.

Affected Models

MacBook Air (all models)
MacBook Pro (all models)
MacBook (2015–2019)

Common Causes

An app holding a sleep assertion — actively blocking automatic sleep
External monitor connected with sleep signaling incompatibility
Power Nap or Wake for network access settings preventing idle sleep
A caffeinate process running in Terminal keeping the Mac permanently awake
Corrupted power management settings requiring an SMC or NVRAM reset

How to Fix It

Check Activity Monitor for sleep-preventing apps.

Open Activity Monitor (Applications > Utilities) and click the Energy tab. Look at the Preventing Sleep column — any app listed here is blocking the Mac from sleeping. Quit that app and test whether automatic sleep works.
Review Energy Saver or Battery settings.

Go to System Settings > Battery > Options. Disable Enable Power Nap, Wake for network access, and Prevent automatic sleeping when the display is off if they are enabled. These settings keep the Mac in a partially awake state that may prevent full sleep.
Disconnect external displays.

External monitors connected via HDMI or USB-C can prevent the MacBook from sleeping, particularly if the display does not properly signal sleep readiness. Disconnect the external display and test whether sleep works correctly. If it does, the display cable or monitor is the cause.
Check for a running caffeinate command.

Open Terminal and run: pmset -g assertions Look for PreventUserIdleSystemSleep entries not associated with a visible running app. If found, stop the process with: killall caffeinate A background caffeinate command keeps the Mac awake indefinitely.
Reset SMC (Intel) or restart (Apple Silicon).

On Intel MacBooks: shut down, then hold Shift + Control + Option + Power for 10 seconds, release all keys, then press Power to start. On M1/M2/M3 MacBooks: a full shutdown and 30-second wait, then restart, resets power management state. Corrupted SMC settings are a common cause of sleep failures on Intel Macs.

When to Call a Professional

Sleep and wake issues are almost always software and settings problems. If the MacBook will not wake from sleep consistently and a full restart is the only recovery, the sleep/wake circuitry may have a hardware fault. Contact Apple Support.