Product review (5 stars): Quest for better sleep. CO2 meter and Air quality meter to guide you in the right direction.
First impression is excellent. All measurements are displayed very clear, manual shows how to operate very clearly.  As for accuracy, close enough for my non-professional usage.

Now for a short story..

I've recently been on a mission to figure out why my sleep quality is so bad. I thought I liked to sleep in, but lately I really want to wake up early, but when I do, I just end up falling back to sleep for another couple hours.

My room doesn't have a ventilation system, just door and window. I usually sleep with both closed. But after seeing the readings this morning, I will be changing that!

According to the unit, good air is less than 700 ppm; normal air less than 1000ppm; poor air above 1000 ppm.

CO2 readings before sleeping (closed door/window) were around 600-700ppm @ ~11PM
CO2 readings this morning around 8am-9am.. 1500ppm!!!
CO2 readings as I write this review with the window closed and door wide open and fan circulating air in the room, 537 ppm.

Contradicting my findings, I did wake up feeling fairly well rested. I think due to the Germ Guardian air purifier running all night on low which I recently started doing to keep particulate matter low (only because my newly acquired Dylos air quality monitor displayed high readings ).

Reasons for keeping window closed is dust and external sounds (cars/trains). As a possible solution for dust, I might look into a special window screen that blocks dust and other particulate matter.

Included is a photo of my first reading in the morning waking up with closed door/window (CO2 reading showing ~1500ppm). Dylos air monitor shown with excellent air quality reading (HEPA air purifier was running all night on low).

UPDATE #1: Night Two
When the room is empty with closed door and window, the CO2 levels drop into the 400 ppm range.
With one person sleeping in the room, CO2 increased to 1000ppm in about 3 hours. Opening a window slightly allowed the CO2 level to drop to around 850ppm. To reduce dust, I placed a Cut-to-Fit Carbon Pad for Air Purifiers in the window opening. This has slowed down the dust particles significantly. Will have to see how effective it is during the day.