I'm not Sqad but here's my halfpenny's worth.
You spend the night lying down breathing shallowly for the most part, allowing mucous to build up so as soon as you sit up everything starts. You are sweating under the bedclothes and not moving much so you stiffen up.
During the day you are walking about, bending, stretching going about your usual daily business and frequently blowing your nose, coughing up mucous, getting fresh air. No chance of a mucous build up or stiffening up, even if you are taking things easy.
Evening comes, you settle down in front of the tv, not moving much, shallow breathing, sweating under that throw......