Cluster headaches occur in cycles: sufferers can experience several headaches a day for weeks or even months on end.

VOA: standard.2009.12.17

Hearst and his partner, Ben Gluckstern, launched a bike-sharing program called NY Cycles in 2009.

VOA: standard.2010.01.31

"We are going on the rise again, but that's due to normal cycles,"

VOA: standard.2009.12.21

Storm frequency,he says, comes in cycles of 30 years.

VOA: standard.2009.12.21