Commonality between these words 2

My friend sent me an email this time. I had only just woken up, and was very sleepy (my head actually dipped down many times, but that helped me with this new problem.) It went like this:

Well, well, well. Good work with the first one, although it wasn’t all that hard, was it? Well, here’s another one. Find out the commonality with these words; the vowels are part of the problem, but not the whole key!

• Swerve

• Ted

• Rated

• Fatter

• Wade

• Gassed

• Scarf

• Qatar

• Trade

• Texas

• Vat

• Tracer

• Waxed

• Fade

• Serf

• Vets

I sent a reply email back to my friend before heading downstairs for a cup of coffee. Can you solve this one? What is the key to his second riddle?