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I have a 2017 5k iMac, running latest Mojave (10.14.6). I've noticed on a few occasions, sometimes shortly after manually sleeping the machine, sometimes in the middle of the night, that the backlight is on but the screen is black.

Power Nap is disabled, according to System Preferences. pmset -g has the following to report:

$ pmset -g
System-wide power settings:
Currently in use:
 Sleep On Power Button 1
 autopoweroff         1
 standbydelayhigh     86400
 autopoweroffdelay    28800
 proximitywake        1
 standby              1
 standbydelaylow      0
 ttyskeepawake        1
 hibernatemode        0
 powernap             0
 gpuswitch            2
 hibernatefile        /var/vm/sleepimage
 highstandbythreshold 50
 displaysleep         3
 womp                 0
 networkoversleep     0
 sleep                0 (sleep prevented by sharingd)
 halfdim              1
 autorestart          0
 disksleep            10

It happened last night. Executing the following command produced the subsequent output, which unfortunately is very wide:

log show --info --debug --last 24h --predicate 'subsystem == "powerd"'

2019-08-24 21:26:43.082610+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Entering Sleep state due to 'Software Sleep pid=101'
2019-08-24 21:26:43.082678+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] connectionFireNotification: 0x0
2019-08-24 21:26:58.623432+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process apsd.84 Released InteractivePushServiceTask "com.apple.apsd-requestkeepaliveproxy-push.apple.com" age:00:00:15  id:77309453878 [System: DeclUser BGTask IntPrevDisp IPushSrvc kDisp]
2019-08-24 21:27:00.621041+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] No AdaptiveWake is requested. InactivityEnd:Sun Aug 25 09:00:00 2019 PowerNap State:0
2019-08-24 21:27:00.621252+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:adaptiveDisplay] Denying sensor tracking as system is not in user-active window. Active window start:'Sat Aug 24 14:00:00 2019' end:'Sat Aug 24 21:00:00 2019'
2019-08-25 05:27:03.992186+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] No need to refresh inactivity prediction: phase=0, start=0.000000, now=588400023.992176
2019-08-25 05:27:04.161976+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] sendNoRespNotification: 0x8019
2019-08-25 05:27:04.163112+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Updating wake start timestamp to 701616146093879
2019-08-25 05:27:04.989453+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Updating wake end timestamp to 701629912693660
2019-08-25 05:27:04.989540+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] vm.darkwake_mode: 0 -> 1
2019-08-25 05:27:05.003542+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:adaptiveDisplay] Denying sensor tracking as system is not in user-active window. Active window start:'Sat Aug 24 14:00:00 2019' end:'Sat Aug 24 21:00:00 2019'
2019-08-25 05:27:05.005255+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] DarkWake from Standby [CDN] due to EC.SleepTimer/SleepTimer:
2019-08-25 05:27:05.016347+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] <private>
2019-08-25 05:27:07.487909+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] sendNoRespNotification: 0x19
2019-08-25 05:27:07.489211+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process powerd.64 Created InternalPreventSleep "com.apple.powermanagement.acwakelinger" age:00:00:00  id:55834617406 [System: PrevIdle DeclUser BGTask SRPrevSleep IntPrevDisp kCPU kDisp]
2019-08-25 05:27:07.489736+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process mDNSResponder.188 Created MaintenanceWake "mDNSResponder:maintenance" age:00:00:00  id:55834617407 [System: PrevIdle DeclUser BGTask SRPrevSleep IntPrevDisp kCPU kDisp]
2019-08-25 05:27:08.487787+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process mDNSResponder.188 Released MaintenanceWake "mDNSResponder:maintenance" age:00:00:00  id:55834617407 [System: PrevIdle DeclUser BGTask SRPrevSleep IntPrevDisp IPushSrvc kCPU kDisp]
2019-08-25 05:27:12.625480+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Updating wake start timestamp to 701615842166072
2019-08-25 05:27:23.860200+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process timed.88 Released NoIdleSleepAssertion "com.apple.timed.settimeofday" age:00:00:11  id:4295009861 [System: DeclUser SRPrevSleep IntPrevDisp IPushSrvc kCPU kDisp]
2019-08-25 05:27:46.458178+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:systemLoad] Activity changes from 0x0 to 0x10. UseActiveState:0
2019-08-25 05:27:46.458179+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:systemLoad] hidActive:0 displayOff:1 assertionActivityValid:0 now:0x187b07  hid_ts:0x180a38 assertion_ts:0x180a38
2019-08-25 05:27:46.458473+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] vm.darkwake_mode: 1 -> 0
2019-08-25 05:27:46.458554+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process iTunes.782 Created NetworkClientActive "com.apple.iTunes.server" age:00:00:00  id:73014486606 [System: DeclUser SRPrevSleep IntPrevDisp NetAcc kCPU kDisp]
2019-08-25 05:27:52.794612+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process powerd.64 TimedOut InternalPreventSleep "com.apple.powermanagement.acwakelinger" age:00:00:45  id:55834617406 [System: DeclUser SRPrevSleep IntPrevDisp NetAcc kCPU kDisp]
2019-08-25 05:27:52.794641+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Summary- [System: DeclUser IntPrevDisp NetAcc kCPU kDisp]
2019-08-25 05:29:01.189045+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process powerd.64 TimedOut InternalPreventDisplaySleep "com.apple.powermanagement.delayDisplayOff" age:08:02:41  id:68719511265 [System: DeclUser IntPrevDisp NetAcc kCPU kDisp]
2019-08-25 05:29:09.157592+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process hidd.107 TimedOut UserIsActive "com.apple.iohideventsystem.queue.tickle.4294968485.17" age:08:02:41  id:38654748171 [System: DeclUser NetAcc kCPU kDisp]
2019-08-25 05:29:09.157733+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Summary- [System: NetAcc kCPU]
2019-08-25 05:31:31.081912+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process coreaudiod.146 Released PreventUserIdleDisplaySleep "com.apple.audio.context984.preventuseridledisplaysleep" age:19:45:43  id:21474877560 [System: NetAcc kCPU]
2019-08-25 05:31:31.081998+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process coreaudiod.146 Released PreventUserIdleSystemSleep "com.apple.audio.context984.preventuseridlesleep" age:19:45:43  id:4295008375 [System: NetAcc kCPU]
2019-08-25 05:31:31.402776+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process coreaudiod.146 Released PreventUserIdleDisplaySleep "com.apple.audio.AppleHDAEngineOutput:1F,3,0,1,1:0.context.preventuseridledisplaysleep" age:08:15:17  id:21474878907 [System: NetAcc kCPU]
2019-08-25 05:31:31.402870+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process coreaudiod.146 Released PreventUserIdleSystemSleep "com.apple.audio.AppleHDAEngineOutput:1F,3,0,1,1:0.context.preventuseridlesleep" age:08:11:12  id:4295009722 [System: NetAcc kCPU]
2019-08-25 05:37:46.575541+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:systemLoad] Activity changes from 0x10 to 0x0. UseActiveState:0
2019-08-25 05:37:46.575541+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:systemLoad] hidActive:0 displayOff:1 assertionActivityValid:0 now:0x187d5f  hid_ts:0x180a38 assertion_ts:0x180a38
2019-08-25 05:37:46.575752+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process iTunes.782 Released NetworkClientActive "com.apple.iTunes.server" age:00:10:00  id:73014486606 [System: BGTask]
2019-08-25 05:37:46.585415+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] chooseStandbyDelay(): lowBattery = false, battery powered = false, capacity=0, lowBatteryThreshold=0; chosen delay=86400
2019-08-25 05:37:46.585771+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Eligible for Standby: 0
2019-08-25 05:37:46.585772+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] standbyDelay:86400 eligibleForStandby:0 elapsedTime:0 gDelta2Standby:86400
2019-08-25 05:37:46.585808+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Entering Sleep state due to 'Maintenance Sleep'
2019-08-25 05:37:46.585880+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] connectionFireNotification: 0x0
2019-08-25 05:38:02.473302+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:assertions] Process apsd.84 Released InteractivePushServiceTask "com.apple.apsd-requestkeepaliveproxy-push.apple.com" age:00:00:15  id:77309453923 [System: IPushSrvc]
2019-08-25 05:38:04.473121+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] No AdaptiveWake is requested. InactivityEnd:Sun Aug 25 09:00:00 2019 PowerNap State:0
2019-08-25 05:38:04.473275+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:adaptiveDisplay] Denying sensor tracking as system is not in user-active window. Active window start:'Sat Aug 24 14:00:00 2019' end:'Sat Aug 24 21:00:00 2019'
2019-08-25 13:38:29.234771+0100 0x24d      Default     0x0                  64     0    powerd: [powerd:sleepWake] Updating wake start timestamp to 702289109010035

A few key details, I think:

  • Power Nap is disabled
  • The log file talks about 'DarkWake` - is that the internal name for Power Nap or something else?
  • The wake is almost exactly 8 hours after the sleep. pmset -g reveals autopoweroffdelay is set to that (28,800s), but the meanings don't match.
  • Others with similar issues over the years (1, 2) don't seem to have found a resolution.

I'm getting no value from it waking (TimeMachine drive is disconnected) so preventing it entirely is as good as stopping it illuminating the screen.

1

iMac have hibernation disabled by default, so they're kinda always on ... also there's proximitywake(nearby apple devices will talk to your mac regardless of its powerstate) i'd change these to a boolean value of 0

sudo pmset -a ttyskeepawake 0
sudo pmset -a tcpkeepalive 0
sudo pmset -a proximitywake 0
sudo pmset -a standby 0 
sudo pmset -a autopoweroffdelay 0
sudo pmset -a womp 0
sudo pmset -a ring 0
sudo pmset -a networkoversleep 0
sudo pmset -a hibernatemode 25 
sudo pmset -a destroyfvkeyonstandby 1

check the manpages though if have any doubts or specific need such as remotely accessing your iMac. specifically the ttyskeepawake option.

man pmset

From what i gather, 'Dark Wake' is actually related to software updates performed automatically by apple while the iMac is not being used(similar to the iphones automatically install updates function) and/or bluetooth connected accessories.

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