inertia / lethargy
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lethargy
A state in which someone has no desire or energy to move or be active; a general lack of drive.
Nach dem üppigen Mittagessen überfiel ihn eine große Trägheit.
After the lavish lunch, a great lethargy came over him.
The noun 'Trägheit' (inertia/lethargy) is derived from the adjective 'träge' (slow, sluggish, lacking drive). The ending '-heit' is often used to form abstract feminine nouns from adjectives that describe a state or quality.
Er konnte seine Trägheit einfach nicht überwinden und blieb auf dem Sofa liegen.
He simply couldn't overcome his lethargy and stayed lying on the sofa.
You can recognize the physics meaning from signal words like 'Gesetz' (law), 'Masse' (mass), 'Kraft' (force), or 'Bewegung' (motion). In this context, 'Trägheit' (inertia) is a neutral technical term and has nothing to do with laziness.
In der Physik ist die Trägheit einer der fundamentalen Grundsätze der Mechanik.
In physics, inertia is one of the fundamental principles of mechanics.
In everyday language, 'Trägheit' (lethargy) is often combined with verbs of feeling or overcoming. Typical phrases are 'seine Trägheit überwinden' (to overcome one's lethargy), 'gegen die Trägheit ankämpfen' (to fight against lethargy), or 'eine gewisse Trägheit spüren' (to feel a certain lethargy).
Manchmal muss man einfach seine Trägheit überwinden und mit der Arbeit anfangen.
Sometimes you simply have to overcome your lethargy and start working.
In the figurative meaning, 'Trägheit' (inertia) is often used to criticize slow, cumbersome systems. Common combinations are 'die Trägheit der Bürokratie' (the inertia of bureaucracy), 'die Trägheit der Verwaltung' (the inertia of the administration), or 'die Trägheit des Systems' (the inertia of the system).
Viele Bürger beschweren sich über die Trägheit der Behörden.
Many citizens complain about the inertia of the authorities.
sleep
'der Schlaf' and 'die Trägheit' can both describe a lack of movement or reaction. 'Schlaf' is usually used figuratively here: someone or something is not attentive or not active. 'Trägheit' more often means a lack of energy, slow reactions, or little drive. This can apply to a person, but also to a system, such as a government office. 'Schlaf' sounds more like an image for complete rest, while 'Trägheit' more directly describes slow or sluggish behavior.
Nach der Arbeit kämpft meine Mutter oft gegen ihre Trägheit und geht noch spazieren.
After work, my mother often fights against her lethargy and still goes for a walk.
Der Schlaf der ganzen Abteilung endete erst, als der Kunde sich beschwerte.
The whole department's slumber only ended when the customer complained.