During the Quest, by talking to captain Bernard you get an Objective to 'Find out if some herbalist could brew up something to fend off Sleep'.
This video shows where to find the herbalist and how to persuade her to brew the potion that could fend off sleep.
When you get the Quest, 3 herbalists are highlighted on map.The one that you need to go it the one near samopesh.
Go the the herbalist and talk to her. She will agree to prepare the potion only if your reputation with her is higher than 70.
if your reputation is below 70 then you will have to increase it.
To increase reputation buy any item and then haggle the price and pay more.
Kingdom Come: Deliverance Statistics For Gamerpillar
At present, Gamerpillar has 1,406,796 views spread across 113 videos for Kingdom Come: Deliverance, with his channel currently having around 18 hours worth of content for Kingdom Come: Deliverance. This is 1.49% of the total watchable video on Gamerpillar's YouTube channel.