I'm so sorry that I've only just seen this!

Each hour is split into 6 rows denoting 10 minutes each.

So each row has this formula which will yield a "2" if those 10 minutes is within the start and end times in the Sleep data sheet.

=COUNTA(FILTER('Sleep data'!$A$2:$B,(D$2+$B5+IF($B5<$B$5,1,0))>='Sleep data'!$A$2:$A,(D$2+$B5+IF($B5<$B$5,1,0))<='Sleep data'!$B$2:$B))

If it doesn't, it will yield a number that is 1 or below. Hope that explains it.