Nelle, that's interesting that babies respond better to higher pitches - but I agree that there is no need to sound like you've just inhaled a helium factory.

When I sing to him I find that he responds really well to lower pitches - but that is probably because he is against my chest and he can feel the vibration more intensely. I make up silly songs - usually variations on a theme of 'what will happen to Euan if he doesn't go to sleep' (will he go in the microwave or get fed to the crocodiles), but our current favourite is one that DP made up - it goes a something like this...

Hey little Euan
when you gonna spewin'
when you gonna pooin'
when you gonna weein'
when you gonna go to sleep...

It even works as a round.