Hi Sara,
While most cases of Tinnitus are caused by hearing loss, this is only the case in about ~60% of all Tinnitus cases. Most other tinnitus is caused by a neuromuscular inflammation, and often times will have a somatic component - meaning that stress, body movements, postural aggravations, etc. can all have an immediate and instant impact on volume or pitch in one or both ears. The "smoking gun" for this particular scenario is the severe "waxing and waning" behavior that this type of tinnitus exhibits. All Tinnitus sufferers, no matter the cause, experience some "waxing and waning" - that the pitch of their tinnitus and its volume might fluctuate from time to time. This is usually gradual and disconnected from any particular act that they can directly cause, which makes it incredibly frustrating. For people who suffer from tinnitus caused by neuromuscular inflammation, the waxing and waning happens rapidly and throughout the course of a single day. For example, you might wake up with quiet tinnitus and then after stretching in your bed, it comes roaring back. Or, in the middle of the day, it might suddenly dip down to a whisper for no apparent reason, only to come surging back.
These are usually signs of somatic processes, and they are normal. This is a "smoking gun" because in studies performed by Dr. Robert Levine, he has shown that this behavior -- the Tinnitus shifting in pitch and volumes rapidly throughout the course of a single day -- is connected with an inhibition or dysfunction of the Dorsal Cochlear Nucleus, the piece of the brainstem that sits at the base of the skull and at the top of the spinal column that filters the inputs from your cochlea.
What does all this information really do for you? It should encourage you that there are physical things that you can do to lessen the impact of the tinnitus in your life and, over time, assemble what I call a "toolkit of tricks" to help you negotiate your tinnitus. For example, do hot showers with lots of steam help lower your tinnitus? What about postural stretching exercises and plenty of sleep in a comfortable bed? Perhaps a course of anti-inflammatory medications might yield benefit for you. These aren't cures, but they do help people to varying degrees.
The best hope for total resolution of neuromuscular inflammatory tinnitus is through physical therapy. The underlying cause of these muscle spasms -- whether it's a TMJ dysfunction through clenching teeth, or a neck spasm through bad sleep -- is long-term chronic behavior, and as a consequence, long-term physical therapy to help you correct the underlying physical problem can truly provide remarkable results. But the big frustration is going to be getting this diagnosed, as more than half of all Tinnitus cases will never be successfully diagnosed (largely because ENT's are generally ignorant about tinnitus' workings). It will leave people afflicted with tinnitus few options other than to self-diagnose through a process of elimination.
In your case, though, I think it's a safe bet to say that if a night guard has significantly helped, you're on the path towards figuring our your own toolkit of tricks and start the healing process in earnest.