Herb to induce sleepiness, eventual death?

[transemacabre]

I need an herb that's available in a temperate forested area in Europe that will induce sleepiness and death in high enough doses. It has to be able to be easily added to either water or food. Side effects such as disorientation, discolored skin, etc. would be a bonus but not necessary.

Googled 'herbs to induce sleepiness', variations, unsure if the results meet my qualifications.