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:
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.
The Welsh Conservatives thank their candidate Sarah Atherton and her opponents for the manner in which the election has been fought.
Plaid Cymru leader Leanne Wood tweets her thanks to candidate Carrie Harper
Wales Governance Centre lecturer joins the debate on whether a turnout of 25-30% - lower than Alyn and Deeside saw in the 2016 Welsh Assembly election - would be considered particularly poor.
BBC Wales political correspondent Daniel Davies at the count in Connah's Quay says the "likelihood" is that Labour candidate Jack Sargeant will be elected to succeed his late father Carl as Welsh Assembly member for Alyn and Deeside.
But he adds that it will be an "interesting dynamic" to see him in the Senedd alongside Carwyn Jones.
The scene at Connah's Quay Labour Club where some of Jack Sargeant's supporters are gathering as the votes are counted in Coleg Cambria.
This is the fourth by-election to the National Assembly for Wales since its formation in 1999.
Val Lloyd was elected as the Labour AM for Swansea East in a by-election in 2001 following the death of Val Feld.
Trish Law was elected to represent the people of Blaenau Gwent following the death of her husband Peter Law in 2006.
And Rhun ap Iorwerth was chosen by the electors of Ynys Mon to represent them after former Plaid Cymru leader Ieuan Wyn Jones stood down in 2013.
Labour by-election candidate Jack Sargeant, seen here speaking to former Rhondda AM Leighton Andrews at the count has tweeted to thank "the community for their support", and his fellow candidates for "the good spirit in which this election has been fought".
Clwyd South Labour AM Ken Skates says the party's campaign has "taken nothing for granted and we have fought for every single vote".
"In Jack Sargeant, we have had a fresh and dynamic candidate, who promises to bring a new perspective to our politics in Cardiff Bay, and our campaign has harnessed these attributes.
Plaid Cymru has thanked its candidate, Carrie Harper, for running "an excellent campaign with a positive message".
Duncan Rees is the candidate for the Wales Green Party.
Mr Rees, who has lived in Wrexham for 23 years, previously worked for the Co-operative group in its membership, PR and community team in Wales.
He is active in a range of voluntary organisations, including the support group for the Wrexham-Bidston railway line. Mr Rees said the Green party "has the ideas, the energy and the policies to provide an agenda for real change that other parties are not offering in Alyn and Deeside".
Donna Lalek is a former teacher and qualified barrister.
She is a community councillor from Broughton and a founder member of charitable and activist group Everyone Matters.
In the Wales Live hustings Ms Lalek called for a second poll on Brexit. "We're now starting to get a flavour of jobs being at risk, prices going up," she said. "We've got to put the terms to the people."
UKIP has not fielded a candidate in this by-election.
The party said it would not stand "out of respect to the late Carl Sargeant".
Last time around UKIP came third in Alyn and Deeside, with more than 17% of the vote.
UKIP's candidate in 2016 was Michelle Brown, who became an AM for North Wales through the regional list.
Plaid selected Wrexham councillor Carrie Harper, who serves the ward of Queensway, to fight the by-election for the party.
Ms Harper is on the council's lifelong learning, homes and environment and democratic services scrutiny committees.
Her campaign focused on the NHS. She had told the Wales Live by-election hustings that health services were being "badly managed".
Labour's candidate Jack Sargeant is among those who have arrived.
Some of his father's friends from Cardiff Bay politics are also here, including Tory AM Darren Millar, political consultant Daran Hill and former local government minister Leighton Andrews.
The Conservatives came second here at the last assembly election, polling 4,558 votes.
Their candidate this time is Sarah Atherton, who is a community councillor in Gresford, the village where she lives.
A former nurse and social worker, Ms Atherton has called for a "root and branch review" of NHS management and leadership, claiming there is a "lot of waste" in the health service.
The most high profile candidate in the race for Alyn and Deeside is the son of the former assembly member for the constituency, Carl Sargeant.
During the campaign he said:
"I will be standing on a platform of being a powerful voice for local people - in the proud tradition of my father".
At 23, the engineering graduate would also be the youngest ever AM.
Prof Roger Awan-Scully from Cardiff University's Wales Governance Centre says:
“The result of this by-election is very predictable. Labour will hold what is a safe Labour seat. A seat which Carl Sargeant won with a comfortable majority even in a poor year for Labour in 2016.
Yet this has been the most extraordinary Welsh by-election I have ever seen. The First Minister and leader of the Labour party in Wales, who showed last year in the general election that he is a formidable election campaigner, has been completely unable to take part in the campaign.
The result of this election will mean that there will be a direct reminder of those circumstances and a reminder of the divisions that run now right through the Welsh Labour party. It'll be in front of Carwyn Jones, in front of everybody in the Welsh Assembly every day. These problems for the First Minister and his party are not going away”.
The first count gets started in Connah's Quay.
The count in Connah's Quay could all be over by as early as 1230 GMT.
Returning officer Colin Everett predicted that, with the low turnout, it could be finished by between 1230 and 0100.
Wales politics reporter
Turnout is expected to be low, according to returning officer Colin Everett.
The chief executive of Flintshire council gave a "guesstimate" it would not be much over 30%.
There has been a postal vote turnout of 61%, which is also down on what is usually expected.
There were 42 polling stations for the by-election, with an electorate of just over 64,000.