deep sleep not recorded

I gave my husband Vivosmart 4 for his birthday on May 3rd.  To this date, the watch did not record a single night with deep sleep.  He sleeps well and feels refreshed in the morning (self-reported). I am very familiar with the watch, have been using it for a while so I don't think this is a user problem. 
Is it possible that some chronic heart problems, such as afib, prevent the watch to record deep sleep???? I.e. if his heart changes the pulse frequently and therefore the watch assumes he is not in deep sleep?

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