There is no special cause of early morning dreams. But the dreams that occur just before awakening are remembered more easily and often than dreams from other periods of sleep.
The superstition probably is related to the fact that dreams that occur just before awakening (generally in the early morning) are more easily remembered than other dreams.
temperature
Condensation of water vapor causes dew.
Water vapour due to condensation in the exhaust pipe.
Rapid eye movement is associated with dreams in the early hours of the morning.
Awesome dreams
Time and space in dreams do not correspond to time and space in reality.
Early Morning Rain was created in 1964.
Early Christmas Morning was created in 1998.
Early Sunday Morning was created in 1930.
Early Morning Hymns was created in 2004.
Early Morning Dream was created in 1972.