Jamieson, Fausset and Brown on Nehemiah 7:3

3. Let not the gates of Jerusalem be opened until the sun be hot, &c.--In the East it is customary to open the gates of a city at sunrise, and to bar them at sunset--a rule which is very rarely, and not except to persons of authority, infringed upon. Nehemiah recommended that the gates of Jerusalem should not be opened so early; a precaution necessary at a time when the enemy was practising all sorts of dangerous stratagems, to ensure that the inhabitants were all astir and enjoyed the benefit of clear broad daylight for observing the suspicious movements of any enemy. The propriety of regularly barring the gates at sunset was, in this instance, accompanied with the appointment of a number of the people to act as sentinels, each mounting guard in front of his own house.