Understanding Spread per Time-of-Day

For a recent project I calculated the average spread per time-of-day for several symbols. To my surprise, all symbols showed increased spread around midnight (GMT+2/GMT+3). I expected JPY, NZD and AUD pairs to be traded more during the Asian session, and EUR, GBP, USD more during the London and NY sessions and hence see the maximum spread at a different time of day for these pairs.

Any thoughts on this?

A few examples below (using ICMarkets EU broker data, averaged over the last 4 years):

Take a look at this and see how it compares - it's tier 1 market liquidity. With exceptions of very high volatility environments like NFPs, usually the two track closely together
https://www.forexfactory.com/attachm...3&d=1641566359
Midnight uK (in your charts that's 2am) is around the lowest liquidity - therefore highest spread) so not a million miles off. However, I suspect ICMarkets is simply hedging their book at 2am London which is what's giving the spike specifically at that time.

Someone suggested rollover causes this. Brokers disable live trading for a few minutes at that time while they reboot/reset(?) their systems. Does that make sense?

definitely possible and could even be both - the T&Cs should tell you what time they calculate the swaps. usually it's midnight but it varies a bit from broker to broker so if it matches with 2am, there's your answer. Thing to note though that's broker specific, not universal. Barclays isn't rebooting at 2am, so it's a retail MM 'feature' not market one.