schlummern
to slumber
To sleep lightly or not deeply; to doze.
Das Baby schlummert friedlich in seiner Wiege.
The baby is slumbering peacefully in its cradle.

to slumber / lie dormant
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to slumber
To sleep lightly or not deeply; to doze.
Das Baby schlummert friedlich in seiner Wiege.
The baby is slumbering peacefully in its cradle.

You can recognize the meaning of 'schlummern' (to slumber / to lie dormant) from the subject. If a person or animal 'schlummert' (slumbers), they are sleeping lightly. If an abstract concept such as talent, danger, or potential 'schlummert' (lies dormant), it is hidden or inactive.
Der Kater schlummert auf dem warmen Fensterbrett.
The tomcat is slumbering on the warm windowsill.
In the meaning 'leicht schlafen' (to sleep lightly), 'schlummern' (to slumber) is a rather poetic or literary word. It sounds gentler and more peaceful than the neutral word 'schlafen' (to sleep).
Nach der langen Wanderung schlummerte er erschöpft auf dem Sofa.
After the long hike, he slumbered exhausted on the sofa.
In the figurative meaning, the structure 'in jemandem/etwas schlummert ...' (in someone/something lies dormant ...) is often used. The preposition 'in' with the dative indicates where the hidden potential or danger is to be found.
In meiner Kollegin schlummert ein echtes Organisationstalent.
A true talent for organization lies dormant in my colleague.
The figurative meaning is frequently combined with abstract nouns. Typical examples are: 'ein Talent schlummert' (a talent lies dormant), 'eine Fähigkeit schlummert' (an ability lies dormant), 'ein Potenzial schlummert' (a potential lies dormant), or 'eine Gefahr schlummert' (a danger lies dormant).
Die Archäologen ahnten nicht, welcher Schatz unter der Erde schlummerte.
The archaeologists had no idea what treasure lay dormant beneath the ground.
to sleep
'schlafen' and 'schlummern' can both mean that someone is sleeping. 'schlafen' is the normal, neutral word for sleep. 'schlummern' refers more to a light, peaceful sleep and sounds softer or a little more elevated. In everyday life, people usually say 'schlafen'; 'schlummern' fits better in a calm scene, with children, or in narrative language. Both words can also be used figuratively when something is present in a hidden way. For example, you can say that a talent 'schlummert' in someone.

Nach dem Mittagessen schlummerte das Baby friedlich im Kinderwagen.
After lunch, the baby slumbered peacefully in the stroller.
Du schläfst morgen besser länger, wenn du heute so spät nach Hause kommst.
You'd better sleep in tomorrow if you're coming home so late today.