n
1 vertical piece between two treads of a staircase 梯级竖板.
2 person who habitually gets up early or late in the morning (as specified) 惯于早起或晚起的人
*an early/a late riser 早起[晚起]的人.

n[C]
early/late riser
someone who usually gets out of bed very early or very late
technical the upright part of a step on a set of stairs
risers [plural]AmE
a set of wooden or metal steps that can be moved from place to place, used for a group of people to stand on
　The school choir stood on risers behind the orchestra.