Crypto trades continuously, yet the depth available at any given price varies enormously through the day. The market is always open; the liquidity in it is not always the same size.
Depth is inventory somebody has chosen to expose
Resting orders in a book are not a natural feature of the market. Each one is a firm sitting ready to buy or sell at a price it has quoted.
Quoting carries risk, because a market maker holds inventory that can move against it between the moment it fills and the moment it hedges.
Wider spreads and smaller sizes are how that risk gets priced. When conditions make hedging harder, the rational response is to quote less, not to quote worse-informed.
Thin hours are riskier hours
During quiet periods there are fewer participants on the other side, so an unwanted position takes longer to offload and costs more to clear.
Related venues are also thinner at the same time, which weakens the hedge. A firm that normally offsets exposure across several markets finds each of them shallower simultaneously.
Reducing quoted size in those hours is a straightforward reaction to that combination, and it happens across venues at once because everyone faces the same conditions.
Human oversight has not disappeared
Quoting is automated, but the automation runs under limits that people set and monitor, and risk teams are not evenly staffed around the clock.
Systems typically operate with tighter caps when supervision is lighter, so an unexpected move outside core hours is met with less capital than the same move at midday.
Weekends compound this, because bank settlement is closed and moving fiat between venues to rebalance becomes slow or impossible until the week resumes.
The consequence is larger moves on smaller flow
Price impact is a function of order size relative to available depth, so an identical order consumes more of the book when depth is low.
This is why sharp overnight and weekend moves are common and often partially retrace once depth returns. The flow that caused them was not necessarily large.
For anyone trading in size, the practical implication is that execution quality is a scheduling question as much as a strategy question, and that stop orders left resting through quiet hours are filled against the thinnest book of the week.
Displayed depth is not committed capital
Resting orders can be cancelled, and automated quoting systems cancel constantly as their inputs change. A book that looks deep is describing intent at that instant.
When volatility spikes, quotes are withdrawn faster than they are consumed, so the depth a trader saw a second before the order was sent may not exist when it arrives.
This is why realised slippage on a fast move routinely exceeds what the visible book implied, and why depth charts are better read as a rough gauge of conditions than as a guarantee of fill.