There'll be more reaction and analysis on BBC Cymru Wales services from 6am. In the meantime, here's a recap of the result.

From the reporting team on this page, nos da.

Jack Sargeant appealed for kindness in politics and life in general after being elected to succeed his late father Carl as the Welsh Assembly member for Alyn and Deeside.

The youngest ever AM at the age of 23, Jack Sargeant increased Welsh Labour's majority in the Flintshire seat.

In his speech following the declaration at Connah's Quay, Jack Sargeant said that being kind was "a message I learnt from my dad".

BBC Wales political correspondent Daniel Davies says Jack Sargeant's victory for Welsh Labour in the Alyn and Deeside by-election was no surprise.

But he says it will be interesting to see how the 23-year-old gets on with Welsh Labour leader and first minister Carwyn Jones, who faces an inquiry over the way he handled the sacking of the new AM's father.

Prof Awan-Scully from the Wales Governance Centre at Cardiff University says turnout in the by-election was "down a bit" on 2016, but was respectable for a by-election "in the middle of winter" on an unusual day of the week.

He said it was a "very clear and decisive victory for the Labour party", with a significant increase in its share of the vote.

"None of the opposition parties are really making any great progress," he said, with a "notably poor performance from Plaid Cymru".

Conservative AM Nick Ramsay and Plaid Cymru AM Bethan Jenkins have also tweeted their congratulations to Jack Sargeant:

David Deans
Wales politics reporter

Speaking before Labour's Jack Sargeant was declared winner of the Alyn and Deeside by-election, Tory AM Darren Millar said his party was never expecting victory.

He added that First Minister Carwyn Jones's absence from the campaign had been "notable".

"He wouldn't be welcome on many people's doorsteps," he said.

Mr Millar said many people who had backed Jack Sargeant were "not voting for Carwyn Jones or Welsh Labour".

Several Labour AMs have congratulated their newest colleague already following his victory in the Alyn and Deeside by-election.

Hannah Blythyn, who represents the neighbouring constituency of Delyn, was one of the first off the mark.

Former Lib Dem AM Eluned Parrott also wished him well:

The new AM is "looking forward to going down to Cardiff Bay and standing up for Alyn and Deeside"... He has said that "we are all still devastated by Dad's death"... “This is a by-election we shouldn’t be fighting. There are questions still to be answered. But tonight is not the night for those questions to be answered.“

Despite the tragic circumstances he added that "there is now an opportunity to start a movement to change politics. Let's go back to traditional Labour values and look after those who can't look after themselves. Let our politics have a kind message - let's be kind to each other, a lesson a learned from my Dad."

Jack Sargeant - Labour 11,267

Sarah Atherton - Conservative 4,722

Donna Lalek - Liberal Democrat 1,176

Carrie Harper - Plaid Cymru 1,059

Duncan Rees - Green 353

Jack Sargeant wins with a majority of more than 6,000

Plaid Cymru candidate Carrie Harper tells BBC Wales the campaign for the Alyn and Deeside Welsh Assembly by-election - prompted by the death of Carl Sargeant - was difficult for all concerned.

The candidates are being briefed on the spoiled ballots at the count in Connah's Quay.

The total number of votes cast is 18,618, including 6,519 postal votes.

Claire Flavin, who went to school with Labour candidate Jack Sargeant in Connah's Quay, says the 23-year-old has matured during the campaign and will make an "absolutely fantastic" member of the Welsh Assembly if elected.

Referring to the death of his father Carl Sargeant, Ms Flavin said: "He's let a tragedy help him grow as a person instead letting it push him down."

Welsh Labour minister Ken Skates tells BBC Wales he hopes turnout in the Alyn and Deeside assembly by-election will reach 20%.

He cites cold weather, the time of year and the unusual day of the week for a by-election as factors pointing to a low turnout.