What causes humans to be sleepy?
The release of the hormone melatonin is responsible for the feeling of sleepiness. It is released by the pineal gland and production starts when the light fades, as it's production is inhibited by light stimulation of the retina, the onset of the production is called dim-light melatonin onset. Artificial supplementation can increase sleep quality and regularity related to some sleep disorders and is often administered as pills or patches.

Adenosine is an inhibitory neuromodulator, ie it inhibits neural activity. Neural activity can be either excitatory or inhibitory and adenosine more strongly inhibits the excitatory neural activity. Thus, increased adenosine levels should decrease excitatory effect of nervous stimuli and make you feel more relaxed &amp; sleepy.

Other hormones &amp; neural pathways may be involved (serotonin and the serotonergic neural pathways for example) as well as other stimuli like other people's behaviours (yawning, stretching etc.).

This page and the wiki entry on melatonin have much more information.

From sleepfoundation:


  Melatonin is a natural hormone made by your body's pineal gland - a
  pea-sized gland located just above the middle of the brain. During the
  day the pineal is inactive. When the sun goes down and darkness
  occurs, the pineal is "turned on" and begins to actively produce
  melatonin, which is released into the blood. Usually, this occurs
  around 9 p.m. As a result, melatonin levels in the blood rise sharply
  and you begin to feel less alert. Sleep becomes more inviting.
  Melatonin levels in the blood stay elevated for about 12 hours - all
  through the night - before the light of a new day when they fall back
  to low daytime levels by about 9 a.m. Daytime levels of melatonin are
  barely detectable.