To sleep on one's watch, to be asleep on guard
He was asleep at a time when he was supposed to be keeping watch. Figuratively: he did not fulfill his role as a guard or as the person in charge; he neglected his duty. (Originally from Isaiah 21:8: '...and I stand on my watch all the nights.')
("The man of the Temple Mount would go around at every watch, with torches burning before him, and every watchman who was not standing the man of the Temple Mount would say to him, \"Peace be upon you.\" It is clear that he is asleep; he would strike him with his staff... that he had slept on his watch" - Middot 1:2; and also: "The north wind blows gently / the ship's light is borne pale / on his watch, weary, the navigator has fallen asleep / only I am still awake" - Didi Menusi / Clock Voice)
Translation
Expression (Hebrew)
יָשַׁן עַל מִשְׁמַרְתּוֹ, יָשֵׁן בַּשְּׁמִירָה
Definition (Hebrew)
הָיָה יָשֵׁן בְּשָׁעָה שֶׁהָיָה צָרִיךְ לִשְׁמֹור בִּשְׁמִירָה. בְּהַשְׁאָלָה: לֹא מִלֵּא אֶת תַּפְקִידוֹ כְּשׁוֹמֵר אוֹ כְּאַחְרַאי. הִתְרַשֵּׁל בְּתַפְקִידוֹ (מְקוֹרוֹ הָרִאשׁוֹנִי שֶׁל הַבִּיטּוּי בַּפָּסוּק בִּישַׁעְיָהוּ כא ח:"...וְעַל מִשְׁמַרְתִּי אָנֹכִי נִצָּב כָּל הַלֵּילוֹת").
("אִישׁ הַר הַבַּיִת הָיָה מְחַזֵּר עַל כָּל מִשְׁמָר וּמִשְׁמָר וַאֲבוּקוֹת דּוֹלְקִין לְפָנָיו, וְכָל מִשְׁמָר שֶׁאֵינוֹ עוֹמֵד אוֹמֵר לוֹ אִישׁ הַר הַבַּיִת שָׁלוֹם עָלֶיךָ.נִכָּר שֶׁהוּא יָשֵׁן חוֹבְטוֹ בְּמַקְלוֹ...שֶׁיָּשֵׁן לוֹ עַל מִשְׁמָרוֹ "- מִדּוֹת א ב, וְכֵן:"רוּחַ צָפוֹן נוֹשֶׁבֶת אַט/ אוֹר הַסְּפִינָה נִשָּׂא חִוֵּר/ עַל מִשְׁמַרְתּוֹ עָיֵף, נִרְדַּם הַנַּוָּט/ רַק אָנֹכִי עוֹדֶנִּי עֵר"- דִּידִי מְנוּסִי/ קוֹל אוֹרְלוֹגִין)