The learned says that it is on account of the senses being exhausted with fatigue, dreams are seen by all creatures. The mind, however, never becomes inactive and hence arise dreams. This considered as their principal cause.
इन्द्रियाणां श्रमात् स्वप्नमाहुः सर्वगतं बुधाः। मनसस्त्वप्रलीनत्वात् तत् तदाहुर्निदर्शनम्॥