shaking sensation

[shakeman]

For the past four days, I have intermittently experienced a strange shaking sensation. It feels as if my bed is vibrating at about 240 Hz. It is mild but disturbing. I know the bed itself is not shaking, because I placed a glass of water on it and observed no effects upon the water. It has occurred every night when I have been on the verge of sleep. When I have noticed it, it has startled me to wakefulness, at which point it subsides. I have also experienced it while coming out of sleep in the morning, but only on one occasion. Today, I felt it while sitting at a table. As soon as I looked up from my work, it subsided.

I am a 25-year-old male. My stress level was recently and dramatically elevated due to recently commencing studying for the bar exam. I have been sleep deprived (~ 5 hours per night) in part because of insomnia resulting from this sensation. I consume caffeine and alcohol moderately.