macOS Sleep-Wake – Fix ‘This Computer Has Been Scheduled to Go to Sleep’ Issue

Airs do go into standby mode more readily than other macs.

• MacBook Air (Late 2010) and later: About standby mode

Assuming its not just the extra time to exit standby mode (and isn't really sleeping anymore), have you checked the startup disk preference as the article above recommends?

You may have to look in the console log for sleep / standby messages in case your mac is entering standby / hibernate earlier than the expected one hour. The pmset command can show you the details of your Air's power management settings. Here's what a Mid 2011 Air looks like with mostly default settings:

mac:~ me$ pmset -g
Active Profiles:
Battery Power       -1
AC Power        -1*
Currently in use:
 standbydelay   4200
 standby    1
 womp       1
 halfdim    1
 panicrestart   157680000
 hibernatefile  /var/vm/sleepimage
 networkoversleep   0
 disksleep  10
 sleep      10
 hibernatemode  3
 ttyskeepawake  1
 displaysleep   10
 acwake     0
 lidwake    1

You can also use pmset to change either the number of seconds before standby kicks in, which is standbydelay; or turn standby off completely by setting standby to 0. Use the command man pmset for details.

Realised I answered a dupe of this here: How can I download something with display turned off (Ctrl+Shift+Eject)?

My answer to that question below:

I always use this combo as well, however this does not immediately sleep my machine in ML.

I believe this is a separate issue at play here. According to Siracusa's ML review, unlike in previous OS X versions, in 10.8 your system is eligible for sleep immediately upon "user inactivity" (the inactivity delay being set in your energy saver settings) unless the currently running apps make "power assertions".

In this case, that key combo is signalling to the OS that "user inactivity" should be triggered right away. In such a case, if your particular app that's downloading stuff does not make power assertions, your system will sleep.

I have come across the same issue generally (not necessarily related to triggering the screen sleep combo). See: Mountain Lion sleeps too aggressively