Use 'somnolent' when describing a person who is so tired they're almost in a state of sleepiness. After hiking for hours, his somnolent expression said it all.
Pro Tip 2/3
Before Bedtime
'Somnolent' is often used when someone is winding down before sleep, not just tired from activity. The dimmed lights made her feel somnolent.
Pro Tip 3/3
Not Negative
Being described as 'somnolent' isn't bad; it's a natural state of needing rest. His somnolent mood was understandable after the long journey.