IH is quite rare. The cause is not well understood, but it is
probably related to a problem in the parts of the brain
that regulate sleep and wakefulness. Recent research
suggests it may be to do with malfunction of the same
systems in the brain that sleeping pills act on. People
who have taken sleeping pills and wake up before they
have worn off, tend to be groggy and confused. This is
how people with IH often feel when they wake up.