I'm surprised there isn't a tag for a character in_bed, under_the_covers, tucked_in; sleeping, reading or otherwise.
Would imply lying. Mutually exclusive of on_bed and (possibly) sitting in/on bed (for any one character).
Would apply to beds of all manner, sleeping_bags, futons and the like. Only requirement is that the bedsheets, blanket or similar be drawn over the body. I'd say at least over the legs, possibly past the navel, else it's merely on_bed.
Not sure if under_kotatsu would count.
Examples:
post #1061734
post #1072870
post #1072869
post #1072855
post #1062419
post #1070452
post #292142
post #1057128
post #1024509
post #1050769
post #1020624
post #1000734
post #999040
post #1074530
Unsure - sitting upright, but legs under covers:
post #1057245
post #1018690
Examples of on_bed:
post #1077161
post #1074637
post #1063670
Updated by jxh2154