It's condensation - stop breathing and it will go away.
Warm wet air meets the cold surfaces of the window and cools to the point where it's fully saturated and dumps the excess moisture as condensation. Left on the windows then mould will form. Even though you can't see it there will be some dampness on any external walls too. Worse behind furniture where there's less air movement.
An open top light - just a bit when the outside temperature allows will help. A dehumidifier for an hour or so during the day will help too - you can hire them. Mine cost about £160.
My bedroom had a relative humidity of 75% and very wet single glazed windows in the mornings - five days of dehumidifier for about two hours a day got it down to 35% and the condensation on the windows was then hardly noticeable. Over the last week or so the RH has crept up to 40% without the dehumidifier.
A simple but effective greenhouse RH meter can be had from a garden centre for about £5.
Ventilation is your best solution... and open windows in bathroom and kitchen when making steam and keep the doors closed.
Good luck...
Peccavi, January 2010
