Sleep Mode Schedule needs the ability to disable it

You‘re welcome

Just to be clear. Turning it off does not always work. If you wake up earlier than your schedule your watch may go back into sleep mode, sometimes going back to sleep mode repeatedly.  Also, even if you put your watch in sleep mode and go to bed early, it WILL NOT record your sleep until the schedule decides you should be asleep.

Like the earlier Mk2, the watch can detect when your asleep. but the Mk3 overrides this with a fixed schedule. As long as maintain this schedule your fine. But if your schedule changes frequently because of things like life, then good luck because this doesn't work.

It looks like problem with your device.
For me disabling sleep mode works good, also sleep recording works well, my MK3i is able to record my sleep even if it starts before sleep time schedule and end it after sleep time schedule ends.

I have Descent MK3i 51mm with latest firmware.

I do not know whether it is really a problem with the specific device, or rather with the physiology of the owner, but personally, I used 6 different Garmin watch models, and while I keep the sleep hours very wide (from 10 PM to 10 AM), in all those years that I am using the watches, it never happened to me that the sleep would not be recorded accurately, actually regardless when I went to sleep or when I woke up. I can go to sleep earlier than the schedule (though it does not really happen much), or wake up much later that the 10 AM (happens actually often, since I use to work in the night), and still the sleep is recorded accurately.

I do not really believe it is a problem of the device, but perhaps it happens because the HR and HRV are not accurately read, or because of some physiological conditions. I.e. because of a bad fit or position, because of vasoconstriction, because of low hemoglobin levels, thick epidermis, high tan or tattoos, or because of a very low HR while sitting at the desk, or perhaps also because the Resting HR and Max HR, or the HRV baseline are not correctly established. Inaccurate result may also occur when you do not wear the watch at least 2 hours before going to bed. Best is keeping the watch on 24/24. 

It is really difficult to tell without analyzing a lot of data.

It could be me, or my device, but there are many people with several thread in this forum expressing frustration with this problem. I too have the Descent MK3i 51mm with the latest firmware and this has always been a problem.

When I spoke to support, they confirmed that what I am experiencing is happening to this watch in general. And that there is NOTHING I could do about it.

Usually I wear this all day. Garmin confirmed this is an issue with no solution yet. You are a lucky one.

Garmin doesn't even acknowledge this issue. I don't understand why, their support has been excellent on other issues I have reported in the past.

Sleep schedule issues are just being completely ignored.