Watchful, a.
Full of watch; vigilant; attentive; careful to observe closely; observant; cautious; -- with of before the thing to be regulated or guarded; as, to be watchful of one's behavior; and with against before the thing to be avoided; as, to be watchful against the growth of vicious habits. "Many a watchful night." Shak. "Happy watchful shepherds." Milton. 'Twixt prayer and watchful love his heart dividing. Keble.
Syn. -- Vigilant; attentive; cautious; observant; circumspect; wakeful; heedful. -- Watch"ful*ly, adv. -- Watch"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for watchful; he lists it under alert, vigilant.
Where he writes about it:
One is vigilant against danger or harm; he may be alert or watchful for good as well as against evil; he is wary in view of suspected stratagem, trickery, or treachery. — from vigilant
A person may be wakeful because of some merely physical excitement or excitability, as through insomnia; yet he may be utterly careless and negligent in his wakefulness, the reverse of watchful; a person who is truly watchful must keep himself wakeful while on watch, in which case wakeful has something of mental quality. — from vigilant
Watchful, from the Saxon, and vigilant, from the Latin, are almost exact equivalents; but vigilant has somewhat more of sharp definiteness and somewhat more suggestion of volition; one may be habitually watchful; one is vigilant of set purpose and for direct cause, as in the presence of an enemy. — from vigilant