It should be a bow of the head only, at least in the United States of America.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“160. … When receiving Holy Communion, the communicant bows his or her head before the Sacrament as a gesture of reverence and receives the Body of the Lord from the minister. The consecrated host may be received either on the tongue or in the hand, at the discretion of each communicant. When Holy Communion is received under both kinds, the sign of reverence is also made before receiving the Precious Blood.”
The GIRM approved for Australia is not as specific, saying to bow, but not “bows his or her head”. It has:
“160. … In Australia standing is the most common posture for receiving Holy Communion. The customary manner of reception is recommended to be followed by all, so that Communion may truly be a sign of unity among those who share in the same table of the Lord. When approaching to receive Holy Communion, the faithful bow in reverence of the Mystery that they are to receive.”
The Australian GIRM can be downloaded as a PDF from
acbc.catholic.org.au/bc/liturgy/200707031933.htm by clicking on “Printable Version”.