Low heart rate notification what does it mean?

[Edword]

Late last night I was almost falling a sleep on my sofa and I got a low heart rate notification on my fit bit for the first time. I got my fitbit on 12 of may this year. "we detected that your heart rate fell below 42 bpm for at least ten minutes" It was 39 for 10 minutes. Since I got my fitbit my resting heart rate is shown an average of 54 bpm. On my Huawei band I had for 4 years before it average 61 bpm ( it may not have done low notifications as only cost 30 was quite basic). I realise wrist devices may not be totally accurate. Looking at the daily heart rate graphs it appears the upper and bottom limit each day is populated by my highest and lowest the lowest is as high as 50 and as low as 35 and often 40 but not had a notification before. I wonder if this is because its has not stayed at this for 10 minutes or possibly because I was sleeping so it would know not to send me a notification.

After the nonfiction I googled this to see what it means.  yesterday I did not eat much as had upset stomach so not sure if this relates or it may be one of the latest times I have been awake since getting my charge 6 I felt almost half asleep as was up very late when I got this message I would normally always be a sleep at this time.   I wonder if low heart rate means low in relation to my normal or in relation to someone of my age or reference group. Looking at a heart doctors youtube it appears this heart rate at low rate even if it is accurate may not be that much of worry in its self but just one of many things to consider. As my last device was my Huawei was more basic I never got this type of notification before so wondered when it appeared 

[JohnnyRow]

On phone app from Today screen
gear icon (top right) > Heart Rate > High & Low Heart Rate
Tells you about the setting, allows you to turn it on or off.  It is based on your resting heart rate and age.  It allows you set your own limits.  Please go there to read more about it.
This is just my own personal guess, but I expect it i mostly of value to someone who been diagnosed with a possible heart issue, and whose doctor would like this kind of information.