1An alarm clock, informally short for 目覚まし時計 - a device that sounds at a set time to wake someone up.
目覚ましを6時半にセットして寝た。
I set the alarm clock for 6:30 and went to bed.
目覚ましが鳴ってもなかなか起きられない。
Even when the alarm goes off, I have trouble getting up.
出張のときは念のため目覚ましを二つ用意する。
When I travel for work, I set up two alarm clocks just in case.
スマホの目覚ましの音量を大きくした。
I turned up the volume on my phone's alarm.
2The act of waking oneself up or staying alert; also refers to something done for that purpose, like a strong coffee or brisk activity.
濃いコーヒーは私にとって一番の目覚ましだ。
Strong coffee is my best way of waking myself up.
目覚ましに冷たい水で顔を洗った。
I washed my face with cold water to wake myself up.
午後の眠気には軽い運動が良い目覚ましになる。
A little exercise in the afternoon works well to shake off drowsiness.
3In some regions, a small treat given to children who wake up early in the morning (also called お目覚まし).
祖母は毎朝、早起きした孫に目覚ましとして飴を渡していた。
Every morning, my grandmother would give her early-rising grandchild candy as a little treat.