Bright red and green lights suddenly flashing from back of watch at night

Hello 9298047.  The lights that you're seeing are the heart rate monitor (green) and pulse oximeter (red) lights.  You are able to turn these features off, though that would eliminate the necessary information for sleep data to be calculated.

Why is it that they only flash very brightly for a short time in the night and not continuously as my heart is beating continuously?

The pulse oximeter (red) requires you to be absolutely still for it to measure, so once you move your arm/wrist just a bit, the red light will turn off.

If you lift the watch off your wrist a bit, I would guess that you will continue to see the green lights flashing.  However, if the contact with the skin is broken, that will tell the watch that you're no longer wearing it, so ot will cease trying to measure your heart rate and the green light will turn off.

For more on this, please review the FAQs below:

Why Does Garmin Connect Show Missing Heart Rate Readings During Sleep?

The Heart Rate Sensor on My Garmin Watch Is Not Accurate - check the "Proper Fit of the Watch" section

Thanks foe your reply Kevin, but this doesn’t seem to explain why at about 3am (not the same time every night)when I am asleep both the red and green start flashing very brightly. They are so bright they actually disturb my sleep. 

Can you please take and upload a video of what you see the next time this happens?  I'd like to see exactly what you are seeing so that I can make sure we're on the same page.

I have the same problem. While I’m sleeping all of a sudden I just get awaken but a big red flash. Even if I put the “do not disturb mode on”. How do I stop it? It’s very annoying. It happens every night. More then once per night. I got the watch last week as a present.

I have the same issue as well. The flash is extremely bright and wakes me up at night. 

Thank you 9298047.  It is hard to tell, but I think that your watch may be a bit too loose.  Try tightening it by one eyelet.  The lights in the video are of normal brightness.  If you still are having issues with waking up due to the lights, you can experiment a bit more with the snugness of the watch or you could also turn off the optical heart rate sensor and the pulse ox sensor off while you sleep.

Thanks Kevin will try this