My first experience with yoga is doing it on my bed first thing in the morning and then at night before I fall asleep. As a beginner, I feel like this gives me extra cushioning and support to my head and neck.
What are the benefits of using a mat over the mattress or is a bed the ultimate mat in comfort and support?
I often do some yoga in bed, I’d say the main advantage is it’s better than not doing it at all, and it can be very nice for certain types of poses. But the floor, whether on a mat or not, has advantages over your bed. For one thing, it’s rather hard to do standing poses in bed, so you’re missing a lot! Another is that on a firm surface it is easier to get aligned correctly in many poses, especially as a beginner.
Perhaps start in bed and move to the floor in the am, and the opposite at night.
I’ll skip over the pragmatic thoughts I have and move to the energetic ones. First though a note - classically there were no mats.
The difference is the connection with the ground. Typically one’s bed is suspended in the air. And while beds, much like people, can take on any of the three doshas, most of those are vatta. There is very little sense of grounding and so the relationship with the earth is, at very least, altered (or reduced or non-existent).
Obviously standing poses and backbends and any other pose that requires stability would be better served on a solid surface with a traction between surface and student. Additionally, depending on what sort of bed you have and what sort of body there’s a chance the spine would not be neutral in supine poses that warrant it. Some beds are obviously better than others.
aystam:
If not for my first influence of “yoga in bed” with pillows and blankets, it’s true I might never have started a routine at all. I have been working in some floor exercises. Try to teach myself a few new poses each week. My sister did yoga for several years and when I went to go visit her in So. Cal. she helped me do downward to upward dogs. Can’t wait to finally afford some classes. I went to a class with her once and felt very overwhelmed by how advanced everyone else was at that time. Now I at least know what the basic asanas look like 
Inner Athlete:
Cushion top mattress and I do many of my restorative, at the end of the day, poses on a microfiber futon, listening to baseball games in the background.
I’ve started a slight back bend move by falling back and walking up and down the wall with my hands in a standing position on the floor. I like that one very much as an eye opener.
I sleep on my mat. But to use a bed for asana practice … that never did occur to me. Maybe I should give it a try.
There is a significant difference. What aystam and InnerAthlete says is absolutely correct as far as a mat being more grounded, offering a more well-rounded traditional yoga.
“Mattress yoga” in a restorative sense helps me achieve things I can’t presently do on a floor mat.
For example, a more defined backbend off the side while I’m elevated in total comfort on pillows in reclined hero pose with arms over the head?
Sound good?