'Sleepy' chicken...Has anyone ever had one?

Let me begin by stating that my chicken has seen a vet, and the vet cannot find a single thing wrong with her...

she is a two year old New Hampshire that I purchased from a local poultry farmer about a month ago. she has given only one egg since being with me. I have four hens, all free range and fed appropriately. 'Ruthie' acts like a normal chicken...and then falls asleep...or at least acts that way. she will either remain standing, or hunker down and close her eyes for about 2-3 minutes, sometimes a bit longer, then get up and resume cleaning her feathers, drinking her water, foraging or playing. I dont understand. Has anyone ever had a chicken like this? She is HEALTHY and does not appear to be egg-bound [as the vet says]. Again, the vet cannot find a single thing wrong with her...'picture of health'. what is going on? Is this a sign of simply being broody? THANK YOU in advance!!