In-game icon
Lie Down is an action performed by pressing the appropriate button when available.
Lying down in a 
Sleep Spot or anywhere this action appears will allow you to replenish your Sleep.
The need of sleep will rise over time and will grow faster if 
hunger or 
thirst are high. If the need for sleep rises a certain point, you will encounter the 
Sleepiness negative status effect, and the hominid will get sleepy which gets indicated by a screen slightly fading black on and off again as if the hominid was struggling to keep its eyes open.
While the screen fades black during the negative status effect, hearing will also be impaired, and the "ping" sound for successful completion of tasks will not be heard anymore (or will be dampened).
Approach a Sleep Spot with an Adult or an Elder that is carrying a 
baby and Lie Down for the
Drop Child action to appear. Press the appropriate button and the baby will crawl down onto the sleep spot.
Lie Down on a Sleep Spot and the 
Evolution action will appear. Press the appropriate button and the menu will open.
Mechanics[]
The icon for Lie Down doesn't always appear. The choice will be there but not always the icon.