I find myself sharing this tip every week. When your Mac has been used for several days/weeks, the consistent opening and closing of applications chip away at your available memory (RAM). Soon you will be out of what is referred to as “contiguous memory”, and the machine will start using the much slower virtual memory which writes info to the hard drive instead of the super-fast RAM chip.
To check if you are out of contiguous memory, open Applications: Utilities: Activity Monitor. Select the “System Memory” tab, and look at the number of “Page Outs”. If this number is greater than zero, you are out of memory. It’s time to restart!