Somnolence (noun)

1. A state of drowsiness or tendency to sleep.
2. A feeling of sleepiness or lethargy.

Origin:

The word "somnolence" comes from the latin word "somnus" meaning "sleep" and "-olence" which means state of or tendency towards, therefore it refers to the state of drowsiness or tendency to sleep.

Examples:

1. He felt a wave of somnolence wash over him as he sat in the warm sun.
2. The medication caused somnolence as a side effect.
3. The somnolence was so strong that he couldn't keep his eyes open.
4. The somnolence caused by the long car ride made it hard to stay awake.
5. The somnolence caused by the heat made it hard to focus on the task at hand.