While playing as Wicker, I went to mine some Ryftstal and happened to stay in 6 Grazers' fog for about 1 day. And for the next 4 days I remained completely sluggish. It seems that sleepiness stacks infinitely on Wickerbottom.
1. Play as Wickerbottom.
2. Go into Grazer's sleepy fog.
3. The longer you stay in the fog, the longer you are slowed down.