查找列值低于某个值的时间戳之间的最早时间

timestamp,y
2019-08-01 00:00:00,872.0
2019-08-01 00:15:00,668.0
2019-08-01 00:30:00,604.0
2019-08-01 00:45:00,788.0
2019-08-01 01:00:00,608.0
2019-08-01 01:15:00,692.0
2019-08-01 01:30:00,716.0
2019-08-01 01:45:00,692.0
2019-08-01 02:00:00,672.0
2019-08-01 02:15:00,636.0
2019-08-01 02:30:00,596.0
2019-08-01 02:45:00,748.0
...

我想做的是,对于这个数据帧中的每个时间段,即下午6点到上午5点之间,我想知道 y

我正在考虑执行以下伪代码:

timestamps = list()
for _, row in df.iterrows():
    found = False
    current = row['timestamp']
    val = row['y']
    if current is between 6 PM and 5 AM:
        if not found and value < threshold:
            found = True
            timestamps.append(current)  

但这看起来相当丑陋,而且容易出错,我想知道是否有一种更简洁的pandaish方法来做到这一点?