Short Answer. A LOT.
Most of my clients are always amazed when they hear how much sleep their little ones are getting vs. what they actually need. I hear a silence on the other end of the phone, to the point where I think the call was dropped thanks to my occasional shotty cell reception.
Most of my client's little ones are getting half of the sleep that they require before they start working with me. They will often say to me, my baby is just stubborn, "spirited", colicky ... etc. Most often it's simply sleeping deprivation, they feel it just as intensely as we do.
Let's all get some sleep, shall we?