Adjective
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experiencing or accompanied by sleeplessness
"insomniac old people"
"insomniac nights"
"lay sleepless all night"
"twenty watchful, weary, tedious nights"- Shakespeare
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engaged in or accustomed to close observation
"caught by a couple of alert cops"
"alert enough to spot the opportunity when it came"
"constantly alert and vigilant, like a sentinel on duty"