Is it permissible to say "O guardian angels, awaken me at such-and-such an hour" before sleeping?

This act is impermissible and constitutes major shirk (polytheism), because it involves supplicating and seeking help from other than Allah and from the absent, like seeking assistance from jinn, idols, and the dead. This is based on the general import of Allah's statement: "And that the mosques are for Allah, so do not invoke anyone along with Allah," (72:18) and "And whoever invokes along with Allah another deity for which he has no proof - then indeed his account is only with his Lord. Indeed, the disbelievers will not succeed." (23:117). This includes supplicating the dead, idols, jinn, and angels. The only exception is seeking assistance from a living, present, and capable person, as in the story of Musa. Among the forms of this major shirk are statements like: "O jinn, take him," or "O jinn of midday, take him," or "O angels of Allah, awaken me," or "O jinn of the house, protect me." The Muslim must be cautious of this and seek help from Allah alone and ask of Him alone, for He is capable of all things. He has said: "Call upon Me; I will respond to you," (40:60) and "And when My servants ask you, [O Muhammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me." (2:186). The Prophet (peace be upon him) said: "If you ask, ask Allah; and if you seek help, seek help from Allah."