OK Pixie, there are two things I'm thinking of here.

First of all...REAL morning sickness is due to low blood sugar. If you've been up especially late the night before, it stands to reason that your body has used up more calories than if you were sleeping, so it's possible that your blood sugar is low the following morning which makes you feel sick. Try taking some juice before you even get out of bed in the morning if you've had a late one, and see if that makes a difference.

Secondly, since night is when our digestive system does a lot of its work, it's possible that by having less sleep, your digestive system isn't getting the opportunity to digest all of the food you've eaten from the night before. Again, in my experience of proper morning sickness, being slightly constipated (meaning the food isn't passing through my system as well as it should) has often made my vomitting worse.

I'm assuming that these late nights don't include alcohol, because obviously a hangover would be the obvious reason, but of course a hangover is just dehydration which again can make you feel/be very sick. Make sure you keep your fluid levels up if you're having a late night as while you're awake, you're using more and producing more urine than during your sleeping hours.

Since some women still bleed at their usual cycle times when pregnant, I'd suggest you take a test just in case, if you haven't already, but other than that, I wouldn't worry too much. The body works in mysterious ways, and it's probably just its way of complaining that you're not letting it sleep enough!

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