Very short cycles that are of practical value for short term trading. Given their short periods, they are notoriously difficult to track, as they tend to be frequently distorted (or even subsumed altogether) by strong trends and random influences in the market.
Given this, they seem to appear and run for a time before vanishing mysteriously, then emerging again in more normal conditions. The weekly cycle is well-documented in the stock and other financial markets, with a psychological basis related to traders having limited appetite for holding their positions over the weekend.