That is correct, and also the power usage in sleep mode can be fairly minimal, which is why the battery lasts so long. In the case of the Kindle, the on/off button doesn't actually switch the Kindle off, but puts it into sleep mode (although the manual just keeps it simple by telling you it is off
). The only time it is actually off is when you reboot it.
You can think of it as a standard sleep and a deep sleep (standard sleep still displays the screensaver, press the power button for a little longer to put it into deep sleep and the screen is blank). It's not fully switched off as when you press the power button to turn it back on, it loads instantly and you don't have to wait for it to boot up.