Dogs cough for various reasons including upper respiratory illnesses such as kennel cough
and canine influenza as well as bronchitis and inhaled allergies. Heart and lung issues can cause coughing and collapsing trachea is also a possible cause. Let me give you some sites to discuss some of the causes which may allow you to rule those out.
You can read about bronchitis below.
You don't indicate nasal discharge
so it probably is not allergies but you can read about them here just in case she is allergic to something like laundry detergent on the sheets. If this is the case, Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours.
You can read about these here.
Kennel cough is normally contracted when a dog has been boarded or kenneled or around a large number of dogs such as at a dog show, dog park or pet store. Here is a website with more information on kennel cough. http://www.thepetcenter.com/gen/kenc.html
Canine influenza is now becoming more prevalent and like it sounds it is a canine flu. Here is an excellent site on it.
Both of these are unlikely since it only happens at night. But a heart or lung issue is a possibility since she may be putting more pressure on the chest area when sleeping at night which is leading to the cough. http://www.canismajor.com/dog/cough.html
Your dog could have a collapsing trachea. Here is a site on rcollapsing tracheas.
Sometimes the way a dog lays can lead to the tracheal collapse and coughing results. My suggestion would be to have her checked over by your vet. In the meantime, stroking the throat can help in cases of tracheal collapse. I'd also try not to reassure her. It can encourage a dog to repeat behavior
that gets it attention.
I hope this information is helpful to you. If you need more information or clarification, please reply and I'll get back to you as soon as possible. If you are satisfied, please take the opportunity to rate.