system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get cold-like symptoms in the mornings

Almost every morning, I wake up feeling like I have a full-on cold. This includes a runny nose, headache, sneezing, etc. By midday, though, they're all gone. What's happening to me?
gpt
Most likely you are allergic to either a pet or dust mites.  It is easiest to rule out, so start with this.

Pet:  Your pet sleeps on your bed.  It's dander/hair is on your pillow/sheets.  In your sleep you breathe in this stuff all night long.  Your body reacts with cold-like symptoms.

Dust mites:  You sleep in your bed, dust mites and dead skin fall off, you breathe them in all night long.  Same reaction


Fix:  Ban pets from the bedroom, wash your sheets and dry them properly.  Double up your pillow cases, washing the outside one every time you do laundry and the inside one every other time.  Vacuum, dust, and clean your pillows (beat them outside, tumble dry on low, or whatever setting they indicate).  

If you still get these symptoms after two weeks, you should see a doctor about A) chronic sinus infection (laying down exasperates it, standing/sitting up drains and relieves pressure) or B) sleep apnea.

Solutions provided in order of likelyhood and cost effectiveness.

Source:  Allergic to cats, dust mites, trees, and grass.  Been there, done that.