Stop for a moment and observe your mind. You will find thoughts upon thoughts rising one after another. The mind is always oscillating. Its fluctuation never stops as it continues even in the dream-state. If you go through Patanjalis Yoga Sutras, you will find how he describes the fluctuating states of the mind.
Patanjali divides them into five catagories. They are Pramana (right knowledge), viparyaya (wrong knowledge), vikalpa (imagination), nidra (sleep) smritaya (memory).
Pramana: it’s the right perception of things, i.e. to understand the things as such. In other words to understand the underlying deep reality is right knowledge. This comes either from deduction or from valid testimonies like sacred scriptures.
Viparyaya: it’s the erroneous perception of things caused by avidhya (ignorance). It is due to this avidhya we suppose ourselves as body and run after physical comfort for happiness. This can be removed when there comes right knowledge.
Vikalpa: its that state of mind where there are merely concepts not the real objects. Concepts of virtue, vice, good, bad etc all are vikalpas.
Nidra: it’s the state of deep sleep where there is no object in connection with the mind. The mind without pratyaya, i.e. without any object to get attached to, is nidra. It is the deep sleep where no dream appears.
Smritaya: the faculty of mind which stores the previously experienced things is memory. In another way not letting escape the experienced matters from the mind is memory.