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I am using a Macbook Air with a 64GB HDD and Mountain Lion installed on it.

When I reboot my Air, it shows that I have 7GB free disk space. But after a couple of hours, the system says my disk is nearly full (less than 1GB free space).
What can I do about this? Is it about Sleep Image?

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Several things can use that much memory - before you reboot get a picture of a few things in terminal by pasting these commands and saving the results.

  • du -m /var/vm/*
  • vm_stat

You could also look at activity monitor and see how much memory on the left and how much swap is being used:

Memory in Activity Monitor

Then, you can see how much RAM you have, how much space will be needed for the sleep image or if some other process is using space. Also, check in the System Information - About This Mac window (the tool lives in /Applications/Utilities - choose Command I and then storage) to be sure you don't also have backups being stored locally.

If you don't need them, you can turn off Time Machine and then turn it back on or use sudo tmutil disablelocal if you like terminal commands and know the risks of sudo and trusting someone on the internet giving advice.

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Likely this is mostly attributed to swapping memory to the drive. If your running applications are using more memory than the amount of RAM installed, stuff will be written out to the disk instead.

Try investigating your memory usage in Activity Monitor. You can reduce the amount of memory used by closing apps or running less apps at the same time, but I suggest you free up more disk space in general as 7GB leaves very little margin to work with.

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