Key Difference: Sleep and slumber are synonymous. Both refer to a state of partial or full unconsciousness. During this period, the body and mind are rejuvenated and are restored. Slumber is sometimes used to refer to specifically a light sleep or a doze.

There is not much difference between the two. Both can be used in place of one another. Sleep is an important activity as it provides the rest to the body. Sleep is not a simple activity as it seems to be. It is a process that involves various steps.

1. The first stage is generally defined as the one in which a person closes his eyes but still does not feel like he has slept.
2. In the second state, irregular periods of muscle tone are mixed with the relaxation of the muscles. These are accompanied by a slow heart rate and then a decrease in the body temperature. This stage can be referred to light sleep.
3. The third stage is also known as a delta sleep. This is a stage of deep sleep. Awakening from this stage get be frustrating or disorientating.

Dictionary.com defines sleep as:-

• To take the rest afforded by a suspension of voluntary bodily functions and the natural suspension, complete or partial, of consciousness; cease being awake.
• Botany:  to assume, especially at night, a state similar to the sleep of animals, marked by closing of petals, leaves, etc.
• To be dormant, quiescent, or inactive, as faculties.
• To be careless or unalert; allow one's alertness, vigilance, or attentiveness to lie dormant.

Dictionary.com defines slumber as:-

• To sleep, especially lightly; doze; drowse.
• To be in a state of inactivity, negligence, quiescence, or calm.
• A period of sleep, especially light sleeps.
• A state of inactivity, quiescence, etc.

Therefore, slumber is also used as a synonym of sleep. However, it sometimes is specifically used to denote a light sleep or doze. Slumber is also sometimes described as the state of consciousness that lies in between the states of awake and fully asleep.

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