In the novels, while it is mentioned that Geralt meditates, this is not a huge part of the narrative.
In The Witcher computer game
In The Witcher, Geralt does not need to sleep because he is tired, but he must meditate in order to allocate accrued talents as well as for any creation of bombs, oils or potions. In order to meditate, he must either light a campfire, a fireplace, or choose to meditate in a conversation (typically with an innkeeper, but some other NPCs also offer this convenience).
The slider to the right of the hourglass is used to select how long (in number of hours) Geralt should meditate. Alternatively, one of the four "time of day" icons can be selected to meditate "until dawn", "until midday", "until dusk" or "until midnight".
Clicking on the hourglass in the centre initiates the period of meditation.
Note: If Geralt is brewing potions (or making other alchemical items), or allocating talents, the hour necessary for brewing potions or allocating the talents is automatically added to the number of hours remaining until the desired time of day. So be careful when trying to make a specific appointment.
cs:Meditace