A funny German expression for saying: to do something without much effort, effortless or ‘I can do it on my head’, ‘to be able to do something blindfolded’ and even ‘to know something by heart’. In German you can say in these cases: etwas im Schlaf können / machen / beherrschen’, which literally means: to do something / be able to do while asleep’. This idiom is mostly used in colloquially speech.
Shooting a goal from this distance is easy, I can do that on my head. = Ein Tor schießen von dieser Entfernung ist einfach, kann ich im Schlaf.
Tom doesn‘t need to learn for this math test, because he can solve this type of equations effortless. = Tom braucht für die Mathearbeit nicht zu lernen, weil er diese Art von Gleichung im Schlaf kann.
We‘ve repeated this song so many times that we know it by heart. = Wir haben dieses Lied so oft wiederholt, dass wir es im Schlaf können.
Where this expression came from:
It’s not quite clear when this German expression was actually used for the first time. However, we know from Shakespeare’s „A Midsummer Night’s Dream“ that there are quite a few thing possible while we are asleep.
The meaning of this phrase origines in the fact that our consciousness is less active or even inactive while asleep. Therefore doing difficult things effortless while asleep is valued so much higher.