It is so comforting to hear that I’m not the only one experiencing this! I’ve been scouring the Internet using search terms like “internal head vibrations while sleeping.” But I’ve not found anything that sounds like what I’m having besides what you & an few others are describing on this thread! I am a 43 yo female & this began about 18 months ago. It happened once, was very frightening, and then didn’t happen again for several months. I have not received a single COVID vaccination but I had the DELTA variant. I got very sick but was not hospitalized. I have since learned I am Vitamin D deficient but am otherwise healthy. I did receive the antibody infusion which I believe saved my life. I have found things online that make we wonder if this is a Long-COVID symptom but it literally ONLY happens while I’m in sleeping position—either falling asleep, in a very light stage of sleep in the night, or upon waking up. It came back with increased frequency about 4-6 months ago & is now happening most nights & every morning without exception. I saw a sleep specialist to discuss this tremor along with insufficient deep sleep (per my Apple Watch, which I began wearing to look for patterns during these vibrations). Nothing showed as abnormal during the times I noticed the vibrations but they always stop when I come out of that lightest phase of sleep to check the time. Just as you mentioned, I have never once (thankfully) experienced this feeling while being fully awake or even upright. Mine originates in the base of my skull so I start to feel it in my jaw & lips first. Changing head position stops it temporarily. I’m no longer frightened as I seem to be okay when I’m awake but the sleep specialist seemed completely stymied & that didn’t make me feel great. I did an at-home sleep study which showed some signs of minor sleep apnea & snoring (my husband says I don’t). But maybe it’s too quiet to hear & is causing this tremor somehow. He’s ordered me an on-site sleep study for more info. Perhaps something will be found there.