Benign paroxysmal positional vertigo (BPPV) was initially described in 1921 and is probably the most common cause of an intermittent spinning sensation. The name of the condition is self explanatory, benign (not serious) paroxysmal (episodic) positional (consistently related to a change in position) vertigo (dizziness, a sense of movement, unsteadiness and commonly spinning). In BPPV a sense of movement is precipitated by certain head movements. This commonly occurs when getting into, out of, or rolling over in bed, bending and standing, or with neck extension. Such dizziness usually occurs within a few seconds though may sometimes not occur for up to 40 seconds. Dizziness, usually subsides within 10 to 20 seconds though sometimes may last for up to a minute. Symptoms always abate even when maintaining the position in which the dizziness was brought on. Sometimes the dizziness may recur when returning from the lying down to the upright position. Dizziness in this reverse situation usually lasts for a shorter period, usually seconds only. On lying down again, if this is what precipitates your symptoms, you will find that the intensity of the dizziness is less and neither does the dizziness last as long. Repeatedly lying down and sitting up results also in ever lessening symptoms.
The cause of this condition relates to small particles of material within the inner ear detaching from one compartment and falling into a small semicircular canal that detects movement. The dizziness in this condition may follow a head injury with damage to the inner ear apparatus, or infections in the ear (eg. viruses) and ageing. In most cases, however, no cause is found.
The natural history for this condition is for a spontaneous improvement in weeks or months but in some patients the condition persists unless treated. Drug therapy is not helpful. Treatment with physical exercises is aimed at moving the small particles of material within the inner ear out of the canal and into a chamber that is not sensitive to movement. This involves repeatedly putting your head into a position which will induce the symptoms as illustrated in the Epley or Semont Manoeuvres (see different headers).
See also http://www.dizziness-and-balance.com/disorders/bppv/bppv.html
for a more comprehensive and in depth discussion. This is reasonably easy to read with some patient friendly application.