I live in the Lakeview Manor and my steward is Ogol. During the night everyone in the house (my spouse, my childrens and Rayya) sleep. The bard doesn't sleep because he not have a bed.
The problem is with Ogol. He doesn't sleep in his bed. I don't know what happend with him. I loaded a previous game, I rebuild the house and hired again Ogol not sleep, so I killed him and hire another steward who was not sleeping. Hire another and neither. Really I don't know what to do. Not that it affects me the steward not sleep, but I wish everything were normal in my house (not to mention the giants and dragons sometimes attack xD).
Even if you have crafted the Main Hall's beds (double bed and two single beds), plus built the Bedroom wing with three beds, having a wife and children plus a Housecarl and Steward seems to cause the Steward to sleep in the chair next to the Main Hall's double bed. What may look like 'sitting' is actually 'sleeping.'
Whoever has been my steward, fully furnished house or not, they don´t seem to sleep at all, and what Valen says seems legit :D They usually go upstairs (where´s the other doublebed) and just stand by the bed all the hours they´re supposed to sleep.
If you haven't built the chair in the upstairs Main Hall, build it and see what happens.
There are a total of six possible beds, with eight sleeping spots, if you opt for the Bedroom, instead of either the Enchanting Tower or Greenhouse.
It seems no matter what, the Steward will only sleep in a child's bed, and only if the house was built fast enough for all beds to be built before the children 'lay claim' to them. You might try ordering the Steward to sleep in one of the beds, and see if that resets him/her somehow.
On PC: Go to the bed where your steward is waiting. Open the console, click on the bed, you will be shown an ID. Type "help" followed by the name of your steward an then press enter. In the following output you will find a line with your stewards name and with his ID. Then type setownership followed by the ID of your steward. Problem: Afterwards you can't use this bed anymore