Is it possible that a pipe is freezing? This is one situation when the problem could occur overnight, and then "fix itself" later in the day.
I assume the 10 units are sharing a single central boiler? The control valve for your unit should then be opening and closing periodically to let hot water run to your unit. When you turn down the heat for the night, there could be a period of a couple of hours in which your unit is not calling for heat, and therefore the valve remains closed and water stops flowing in your unit. This could be enough time for a pipe to freeze.
If this is happening, one "fix" would be to somehow improve the insulation between the pipes and the cold exterior walls. Other solutions could involve changing the control system so that water does not stop flowing for such a long time.