Some people turn off their Macs whenever they aren't in use, either to save electricity or simply out of habit. Others leave them on all the time, on the basis that sleep mode uses a trivially small amount of energy and enables you to get back to work more quickly. (I fall into the latter group, turning off my Macs only when I go on vacation, or when for some other reason I expect to be away from them for more than a day.)
If you leave your Mac on all the time, you may find that over a period of days or weeks, its performance slowly degrades. One common reason for this phenomenon is memory leaks (see RAM Usage, page 76), but other kinds of bugs can also lead to excessive RAM and CPU usage that gradually bogs down your system. In addition, as you use your Mac it may create virtual memory swap files on your hard disk if you run low on physical RAM; the more of these files actively in use, the slower your computer runs.
When you begin to notice that your Mac doesn't feel as peppy as usual, try restarting (by choosing Restart from the Apple menu). If you use lots of resource-intensive applications and have a slower machine with comparatively little RAM, you may need to restart as frequently as every day; if you never notice any slowdowns, once a month may be adequate. You be the judge.
I talk more about keeping an eye on potential performance problems later, in Chapter 8.