2 1/2 yr old daughter waking up suddenly crying

My 2 1/2 year old daughter, for the past 3 nights in a row, is waking up in the night crying and afraid.  She has never done this. When its bedtime I lay her down in her crib and kiss her goodnight.  There's a cool mist humidifier in the room, its very quiet, and it provides a little light in the room instead of using a nightlight.  My 3 1/2 yr old son sleeps in the same room in his own bed.
For the past 3 nights she wakes up after about 4 hours and cries, yelling for "Daddy."  Normally, me, as the mom, have been the primary nurturer, although Dad is too, but it's always been kinda like "mommy is the first source of comfort."  So her yelling for Daddy is very different, but I've just thought maybe she wants the protection that Daddy offers.  In any case, we lay her back down, rub her head or back, and calm her down and leave the room.  And less than 10 minutes later it happens again.

I don't understand why this is happening.  My daughter has always gone to sleep on her own and slept just fine.  I feel like she may be having nightmares, waking up screaming for Daddy but she won't go back to sleep unless one of us is in the room, and if we leave, she wakes up crying again and very upset.  I had the hall light on last night when I went in the room and when I turned it off she said "light on?"  So I'm wondering if she is afraid of the dark, although she goes to sleep in the dark!

Please help me, I don't know what is troubling her.

[Kevin Kennedy, Ph.D.]

This is very normal - many children this age go through a time when they wake at night. It is important that you not develop the habit of remaining with her while she falls back asleep. You did the right thing by conforting her and reassuring her. Then leave the room and let her go back to sleep on her own. If she requires a little more light, that's fine. Nothing is troubling her. This is simply part of the process whereby children's sleep patterns mature.