Change External Oscillator to Internal while running

Projects General Guidance
1

Hi!
I would like to know if it is possible to change the arduino from using the external to internal Oscillator while running.
Can this be done before sleeping so to reduce power consumption?
Thank you again!

2

athtest:
Can this be done before sleeping so to reduce power consumption?

Isn't the clock stopped while sleeping anyway?

Also, make sure you've read this for more power saving options.

3

You can't switch from crystal to internal RC oscillator on the fly. You can change the system clock rate on the fly by changing the oscillator prescale (CLKPR register). You can set it as slow as 1/256th speed.