Overnight And Pre-Market Liquidity

Regular trading hours (9:30am–4pm in the US) concentrate the vast majority of participants — retail traders, institutions, market makers — into one continuous session, which is precisely why liquidity there is deep. Pre-market and after-hours sessions, and the overnight gap in between, have only a fraction of that participation: fewer market makers are actively quoting, retail order flow is thinner, and many institutional desks are simply not active. The result is that the same order size which barely moves a stock at 11am can move it noticeably at 7am.

A useful way to picture this is a store that's fully staffed and full of shoppers during the day, but run by a skeleton crew overnight: the same customer request takes longer to fill and gets a worse price, not because the goods changed, but because there are far fewer people around to absorb the request.

Quoted bid-ask spreads during pre-market sessions are routinely several times wider than the regular-session spread for the same stock, and price impact per share traded is correspondingly larger. This matters directly for event-driven trading: earnings are frequently released before the open or after the close specifically to let information digest overnight, but anyone trying to react immediately in the pre-market session pays a real liquidity premium for that speed, often giving back a meaningful share of the apparent edge to worse fills.