Power managment - sleep mode vs shutting down

Hello.

I'd like to know if it's better to save power to use sleeping mode or to shut down the device (or use auto shut down).

In my understanding, in sleep mode the eReader doesn't use much power. Booting must use some power, isn't it?

In your opinion, is it worth it to completely power off the device?
Would an auto shutdown after an hour for example, be interesting to save power and have better battery life?

Thanks