Many of us spend lots of time tapping out words on a keyboard.
Now a team of M.I.T. researchers thinks that how we type might reveal hidden information about the state of our brains.
They designed a study in which a computer could detect which subjects were alert and which were fatigued, just from their typing.
The researchers focused on how long any particular key was depressed.
They designed a program to evaluate the keystrokes, and ignore the actual content being typed.
For the study, 14 volunteers, seven women and seven men, had to type a Wikipedia article when they were fully alert.
Then they were woken up about 70 to 80 minutes after they'd fallen asleep, when they were in deep slumber.
They then had to type out another article while still drowsy.
The result: how long they held down the keys varied much more when they were sleepy than when they were alert.
The research will be published in the journal Scientific Letters.
In their write up the team says that such a system could be useful as a safety feature on software programs for night shifts, to identify users who have become incapacitated by sleepiness.
A variation on the program could help diagnose or monitor neurodegenerative diseases such as Parkinson's by tracking changes in someone's ability to manipulate the common keyboard.