I look at MyAir every morning (been on CPAP for about a month). I also load the data from the SD card into both ResScan and Sleepyhead software.
Here is what I have observed about the circadian rhythms of the machines and software:
A "sleep day" spans from noon one day until noon the next day. The date is taken from the afternoon date.
example: The 24 hour period identified as May 20 goes from noon on May 20 until 11:59 am on May 21.
The CPAP usage for the day is the total of all the time the machine was running in that 24 hour period. Let's say that on May 20 you take a one hour afternoon nap, then sleep six hours over night, and take another one hour nap at 10:00am on the morning of May 21. All of that counts in the same day for a total of 8 hours sleep on May 20. At noon on May 21 a new day begins.
Oh yeah... the time stamp is based on the clock on your CPAP machine. They don't seem to be smart enough to account for daylight savings changes (or maybe they're too smart to skew the data with DST changes), so it might be an hour off some time during the year depending on if your DME set the clock on daylight or standard time. In my opinion it's not worth fussing over, just don't fret if you went to sleep at 1 AM and the data report says it was midnight.
When you look at the data on MyAir it will all be summarized into your scores for that whole day. If you checked MyAir before the morning nap and came back later you would see the nap time was added into your statistics for the day.
Note that MyAir data seems to be updated about an hour after you turn off the machine. If you look up MyAir before the web site updates, then you will see whatever data it had from the previous update. In the example above, if you checked MyAir when you woke up in the morning you would only see the one hour from the afternoon nap.
In my experience (without an afternoon nap), if I go pull up MyAir right after I get up in the morning but before the machine has updated the website, what I will see is a message saying "you didn't use your CPAP machine last night". So it seems to be pretty good about not showing the previous night's data by accident.
Also the MyAir dashboard identifies the date of the "sleep day" you are looking at, and as you may already have noticed there is a little arrow on the dashboard that allows you to scroll back and forth through the two weeks of data stored by MyAir.
p.s. Why use both programs? Glad you asked... Originally I loaded both programs with the intent of trying them out for a week and then choosing one. They both show the same detail information and allow drill down to a very deep level into the data.
It turns out that I like the daily display better on Sleepyhead, but I like the summary graphs better on ResScan. So I continue to use both. All three if you count MyAir.
If I had to choose only one of the two programs today I would probably choose ResScan. That opinion goes back and forth over time. Both are good programs.
PSA: Always write lock your SD card before you insert it into the computer and remember to unlock it before you insert it back into your CPAP machine.