For those of us who may wake up to loud ringing, it may have to do with 2 things. One is the lack of good REM sleep. The other is a phenomenon called 'awakening response' and for some people (like me) this can bring a loud T when waking up, especially when sleep is shallow and not restful, lacking REM sleep.
What is awakening response? It has to do with the brain upon waking up trying to monitor our vital signs, much like when we switch on a complicated machine (such as an aircraft) and all the sensors are activated to give reading to the central monitoring system. If a certain sensor is missing or giving alarming signal, the missing is highlighted and alarm or bell rings to bring to the attention of the monitoring system.
Likewise, when we wake up, our brain will monitor and check our vital signs. Unfortunately it has mistaken that tinnitus ringing is a vital sign. Upon sensing it missing, it zooms on it and recreate the pseudo or phantom sound/ringing in the ears.
Perhaps a doctor who posted on this forum earlier can explain it more technically. Here is what he wrote:
"I have always felt it to be due to the role of the reticular formation in tinnitus perception. The reticular formation is a primitive structure in the brain stem. It's a "vital sign monitor." When you wake up, the RF checks to see whether or not you are alive! It looks for heartbeat, respirations, blood pressure, temperature - basic bodily signs. It does not care whether or not these signs are good or bad - just that they are present. And as far as the RF is concerned, tinnitus is part of "expected" body function in an individual suffering from severe intrusive tinnitus. So when you wake, your RF checks to see whether or not tinnitus is present. And if it doesn't immediately find the tinnitus, the RF seeks it with a vengeance - - especially in the post-nap state, when there has been inadequate REM sleep and when, therefore, the RF is on already "high alert" that something might not be quite right. Theoretically - very theoretically - this RF hyperactivity in seeking tinnitus, which in a person with severe intrusive tinnitus it equates with life, results in temporarily markedly augmented tinnitus perception."