A death row inmate in the southern US state of Arkansas has been granted a new trial after one juror during the original hearing was caught sending Twitter messages and another juror fell asleep.

The Arkansas Supreme Court on Thursday overturned the death sentence for Erickson Dimas-Martinez, 26, and ordered that he be tried again.

"Because we conclude that the one juror sleeping and a second juror tweeting constituted juror misconduct, we reverse and remand for a new trial," the court said in its ruling.

Dimas-Martinez was convicted last year on charges of aggravated robbery and capital murder for the December 2006 robbery and slaying of a 17-year-old boy.

In their appeal, Dimas-Martinez's lawyers complained "that one juror fell asleep during the guilt phase of the trial, a fact that was brought to the circuit court's attention."

Furthermore, "a second juror was posting on his Twitter account during the case and continued to do so even after being questioned by the Circuit Court, as evidence of juror misconduct that calls into question the fairness of his trial."

On the day of the sentencing, the juror posted the following message on his Twitter account: "Choices to be made. Hearts to be broken... We each define the great line."

When defense attorneys pointed this out and claimed he was commenting on the proceedings, the judge spoke to the juror but was convinced he had not spoken about the case, so did not dismiss him from the jury.

"Don't twitter anybody about this case. ... don't twitter, don't use your cell phone to talk to anybody about this case other than perhaps the length of the case or something like that," the judge ordered.

But the juror kept tweeting and broke the rule again when, 50 minutes before the verdict was announced, he tweeted: "Done." The defense noted that one of the juror's followers on Twitter was a reporter.

Posting that message was "a flagrant violation of the Circuit Court's instruction against twittering," the defense attorneys argued.

Dimas-Martinez's lawyers had urged that a mistrial be declared, but the judge denied the motion.

The Arkansas Supreme Court however agreed Dimas-Martinez deserved a new trial.

"Because of the very nature of Twitter as an on online social media site, a juror's tweets about the trial were very much public discussions," the state Supreme Court concluded in its ruling.

"Even if such discussions were one-sided, it is in no way appropriate for a juror to state musings, thoughts or other information about a case in such a public fashion."