I've also tried opening the display settings, holding down the alt key, and clicking 'Detect Displays'. But now the only way to consistently fix the issue is to restart the MacBook. It used to be that when this happens, I manually put the MacBook to sleep and wake it up again, and voila, it would detect the external display again. I use the MacBook at home as well as at work I've tried different ports on my computer, different cables, and different monitors the issue is with the MacBook itself. I'm using a Mini DisplayPort to DisplayPort cable. I come back, and no matter what I try, the only way for the MacBook to detect the external display again is to restart the computer! Apple has to fix this, my MacBook is hardly usable like that Failure code:: 0x01390875 0x0000001f. It just prevents the random shutdowns/reboots. But this is not a fix as the Mac will not really 'sleep' anymore and battery drains faster. Here's my situation: I have an external monitor hooked up to to the MacBook's display port, I walk away from my computer for a few minutes and it goes to sleep. As suggested on Reddit, a workaround is the following: sudo pmset -a standby 0. ( Update: Now running macOS Mojave 10.14.5 on a 13-inch 2018 model, and this is still a very annoying issue). Sleep is not available with Family Setup.I have a MacBook Pro (Retina, 13-inch, Early 2015, macOS High Sierra 10.13.6) which fails to detect an external display after is has woken up from sleep. If you pair your Apple Watch after setup, you can still turn on Track Sleep with Apple Watch later. This option is available during setup if you've paired your Apple Watch with your iPhone beforehand. Track Sleep with Apple Watch: Wear your watch to bed to track your sleep.Getting rid of this is easy, although the solution is a little quirky. This usually happens when you’ve connected your Mac to other wifi networks in the past. Re-joining a network every time you wake up your Mac is one irritating thing. Wind Down Shortcuts: Add shortcuts that will help you take time to relax before bed, such as turning on a Home scene, listening to soothing playlists, or using your favorite meditation app. When you wake your Mac up from sleep, you might find that it disconnects from the wifi.Tap People or Apps under Allowed Notifications and select who or what can notify you while Sleep Focus is on. In the Settings app on your iPhone, tap Focus > Sleep. Sleep Screen: To reduce distractions, iPhone can simplify your Lock Screen and turn on Sleep Focus at your scheduled bedtime. You can also give certain people or apps permission to notify you.Bedtime and Wake Up times: Set when you'd like to go to bed and wake up.Sleep Goals: Set the number of hours you'd like to spend asleep.Most users do not see their screen save under default settings. By default I believe your Mac is set to have the same timer for putting the display to sleep and for the screensaver to kick in. Follow the onscreen prompts to establish: The Mac system itself can also go into a low power sleep mode.Tap Get Started under Set Up Sleep, then tap Next.

Mac sleep timer reddit