The ocean has tides, which are periods of high
water twice a day alternating with periods of low
water twice a day. Each full tide (high + low
tide) takes about 12 hours. So, at night time,
actually, both high and low tide occur. It's
likely that during the times that you've observed
the ocean at night, it has been high tide, but
it's not always the case. The tide is caused by
the gravitational pull on the oceans from the moon
and the sun.
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