The mechanism
Markets run on human and institutional calendars, and those calendars are not uniform. The daily open is when orders accumulated overnight get processed. The daily close is when institutions square risk and mark their books. The weekly close is when P&L gets evaluated and positions get adjusted for the next week. Options expirations, futures rolls, and index rebalances land on fixed dates. Payroll flows, fund inflows, and reporting deadlines all have a schedule.
Because the causes of order flow are scheduled, the order flow itself is scheduled. Liquidity and volatility cluster at the opens and closes of sessions because that is when the institutional reasons to trade are concentrated — not because of anything price is doing. The rhythm of the market is the rhythm of the economic activity behind it, stamped onto the tape.
It is measurable, and it has been measured
This is one of the most thoroughly documented regularities in markets. Harris (1986), studying transaction-level data, found systematic weekly and intraday patterns in returns — the trading day is not statistically flat; specific parts of it behave differently, repeatedly. Decades later, Heston, Korajczyk and Sadka (2010) showed intraday patterns are not just noise but are persistent in the cross-section: returns in a given half-hour of the day predict returns in the same half-hour on subsequent days, a periodicity consistent with scheduled institutional trading. Two independent studies, different eras, same conclusion: the clock matters.
What it gives the trader
Cyclicity tells you that when is information, not just what. The same setup at the London-New York overlap and at the dead of the Asian afternoon are not the same setup, because the order flow available to confirm or reject it is different. It is the structural reason session timing, the daily and weekly opens, and expiration calendars deserve a place in a model — they mark the moments when the market's fuel supply spikes.
You cannot eliminate cyclicity. Human economic activity is cyclical. As long as institutions operate on calendars, the calendar will be visible in the flow.