I understand that the body is a complex organism and that it feels tired so that we sleep and recuperate as necessary. I also understand that the exact process for this may not be known. But can it be measured? If we had the right tool could we measure on a cellular level how “tired” the body is? If I nap 10 minutes at dinner time I cannot fall asleep until hours after my usual bedtime (I get that this may to some extent be individual), I wonder how this messes with the “measurement” of tiredness in my body.