The overnight tends to produce narrower ranges and cleaner trends within them, punctuated by sharp moves at fixed times: Asian opens, China data, the European open, UK and euro-area releases, and the 8:30am Eastern US numbers.
Overnight highs and lows are watched as reference levels by day traders because they mark where a thinner market found sellers and buyers.
Example: a trader arriving at 9:00am marks the overnight high and low on ES; the first 30 minutes of rth frequently trades to one of them before establishing direction.
Related: eth, rth, globex, opening-range, economic-calendar