Condensation on UPVC bedroom Window.

Every Morning when i open the curtains i am unable to see through the window as it is so much condensation on it, i have noticed also that as a result the windowsill is starting to go a bit moldy (however this does rub off). It does clear after about 30 mins if i open the window.
What could be causing this? is there anything i can do to fix is?

Any ideas?

Cheers

Add ventilation.

//wiki.diynot.com/building:condensation_in_houses

Thanks for that. Much appreciated