Chart above shown the average gwei on hourly. we can see on 1600 utc the gwei is the highest on all day. Hence farmer can avoid these times to avoid high gas fees.
From the chart of hourly number of claimed by alcx farmers. Its correlated to the gwei on the chart above. The number of claimed is the lowest on the highest gwei time which is when 1600 utc. Meanwhile the highest number of claimed is on 0500 utc. Farmers claimed reward at this hour is totally make sense for the low gwei during that time.