Explanation:

- Insomnia is a condition where a person has trouble falling or staying asleep.

- Option 1: Restlessness - This generally refers to an inability to stay still or calm. It often conveys unease and can be present while awake or asleep.

- Option 2: Sleeplessness - This synonym directly matches the lack of sleep aspect of insomnia.

- Option 3: Agony - This describes extreme physical or mental suffering, not linked to sleep.

- Option 4: Headache - This is a pain in the head or upper neck region, unrelated to sleep disturbances.

- Correct Answer: Option 2 - Sleeplessness is the nearest synonym to insomnia.