Why FX Traders Watch the Clock: Sessions, Overlaps and the 4pm Fix

Here is a small oddity of the foreign exchange market: it never really closes. Somewhere on the planet, a bank's currency desk is always open, and trading passes around the globe like a baton in a relay race. There are no designated market makers sitting at the centre of it all. Instead, activity simply moves across time zones as the day unfolds.

That matters more than it might seem. When the world's currency desks are awake at the same time, more people are quoting prices, more orders are being filled, and the market can absorb larger trades with less disruption. When they are not, the same trade can leave a bigger mark. So let's walk through the three ideas that do most of the work here: session overlaps, the daily rollover of value dates, and local fixings such as the famous 4pm London rate.

Sessions are not rooms — more like waves

People often talk about the "London session," the "New York session" and the "Tokyo session" as if they were separate buildings with opening and closing doors. In practice, they are overlapping waves of activity. The New York session, for instance, is generally described as running from 8am to 5pm Eastern Time, and it overlaps with London between 8am and midday Eastern Time. That stretch is widely regarded as the most liquid part of the day — and the start of the New York session tends to be livelier than the hours that follow.

Liquidity, by the way, is one of those words that sounds technical but is easy to picture. It is simply how easily you can buy or sell something without moving the price much. A deep, liquid market is a crowded fish market at noon: plenty of buyers, plenty of sellers, brisk haggling. A thin market is the same fish market at 4am: your single purchase can shift the price.

The numbers involved are genuinely large. Turnover in over-the-counter foreign exchange markets averaged $9.6 trillion per day in April 2025, according to the Bank for International Settlements' Triennial Central Bank Survey — up 28% from $7.5 trillion three years earlier. Spot transactions accounted for $3 trillion a day, or 31% of the total. And trading remains concentrated in a handful of centres: sales desks in the United Kingdom, the United States, Singapore and Hong Kong together handled around 75% of global foreign exchange trading. The UK alone accounted for roughly 38%.

When the waves cross, the market breathes deeper

Why does overlap matter so much? Because price discovery — the process by which buyers and sellers collectively settle on a price — works best when lots of independent views meet. During the London–New York overlap, European and American desks are both active, and major pairs such as EUR/USD tend to see their deepest liquidity of the day. Spreads, which are the small gap between the buying and selling price, often narrow when competition is fiercest, which lowers the cost of trading.

Research on currency markets supports the intuition. A study of hourly exchange rates across the six most liquid currencies found what the authors described as a strong "overlapping times effect" in intraday patterns. In other words, the market's pulse isn't flat — it rises and falls with the working day.

There is also a pattern at the edges of the day. A separate study of the Moscow Exchange, which extended its ruble–dollar trading hours several times between 2005 and 2013, found that the extensions significantly increased both volatility and volume — with the biggest jumps appearing right after the market opened, and the smallest effect at the close. The researchers suggested this was because longer hours attracted informed traders more than liquidity providers. That is a useful caution: more trading hours do not automatically mean a calmer, more comfortable market. Sometimes they mean a livelier one.

Incidentally, the fact that exchange operators have to study the effects of moving a closing bell tells you something about how rarely the plumbing changes. Market structure is not remodelled often.

The 4pm fix: one minute, one price, a lot of money

Now to the part that surprises many people new to currencies. A great deal of global finance does not care what the exchange rate is at, say, 11:37am. It cares what the rate is at one specific moment.

The best-known of these moments is the London 4pm "fix" — a benchmark calculated at 4pm London time and published by WM/Reuters, now part of FTSE Russell, an LSEG business. These rates are used to value portfolios, calculate fund net asset values, and settle index and bond contracts that need to express many markets in a single currency. It has been around for more than 30 years, and its methodology is aligned with the IOSCO Principles for Financial Benchmarks. Since November 2024, the 4pm London rate has been designated a Critical Benchmark under the UK Benchmark Regulation.

Here is the wrinkle. For years, the fix was calculated over a one-minute window centred on 4pm, using snapshots of trade and order rates at one-second intervals and taking the median. Because so many portfolio managers wanted to trade exactly at that price — to minimise the gap between their portfolio and the index they track — activity clustered heavily around the window. The Financial Stability Board's Foreign Exchange Benchmark Group, which reviewed the process, documented that trading volume through the fixing window was at times up to ten times larger than at other times of day.

That concentration created uncomfortable incentives. Dealers accepted fixing orders at a price that was not yet known, often without a fee or spread. The group recommended widening the calculation window, using more price sources, and charging for fixing transactions transparently. WM subsequently widened the window to five minutes and added price feeds. As one regulator put it, if the fixing service is not directly and transparently charged for, the incentive for inappropriate behaviour remains.

Rollover: the quiet daily housekeeping

There is one more clock to know about. Foreign exchange trades typically settle two business days later, a convention known as T+2. As each trading day ends in a given centre, the value date for new trades rolls forward — a daily housekeeping ritual that can nudge short-term funding and swap markets. FX swaps, which combine a spot transaction with a forward at a later date, remain the most traded instrument at $4 trillion per day, and most are short-dated, up to seven days. They are the workhorses of currency funding, and they are sensitive to exactly this kind of calendar mechanics.

The practical upshot: a trade's cost can depend on when in the global day you place it. Liquidity is not a constant, and neither is volatility. Around major fixings and session opens, both can spike.

What this means if you are not on a trading floor

If you are changing money for a holiday or sending funds abroad, you will not be setting a benchmark. But the same logic explains why the rate you are quoted at 11pm on a Sunday may differ from the one available mid-afternoon on a Tuesday, and why banks and brokers quote wider margins at quiet hours. It also explains why the timing of a large international transaction can matter more than people expect.

One honest caveat: rules differ by jurisdiction. Benchmark regulation in the UK is not identical to regimes elsewhere, and local exchanges set their own hours. If timing or pricing matters materially for you, check the specific terms your provider operates under rather than assuming a global standard.

The currency market, in the end, is a creature of the clock — busy when the world is awake together, quiet when it isn't, and briefly very crowded at four o'clock in London.