Prime Minister James Callaghan likes to be in bed by 10:30 p.m. and make an early start on the morning papers the next day. Thursday night even the prospect of post-midnight returns from a barometer byelection in Scotland did not disturb his routine.

Callaghan had every reason to relax. He woke up yesterday morning to find that his Labor Party had beaten back a Scottish National Party challenge in a Glasgow seat by a comfortable margin of 4,552.

Callaghan, a politician to his fingertips, now knows he has won some valuable breathing space. Another threat to his plans for choosing - rather than being forced into - a general election date has diminished.

One of the biggest clouds on Labor's political horizon is Scotland. Callaghan's party holds 40 of the 71 seats there, but the Nationalists have been cutting deeply into Labor's strength.

A few weeks ago, they thought they could take the Glasgow seat from Labor. By election day, the experts figured the race was neck and neck. When the votes were counted, Labor's Donald Dewar had collected 45 percent and the Nationalist, Keith Bovey, only 33 percent.

So the 11 Scottish Nationalists in Parliament stay at 11. More importantly, from Callaghan's standpoint, the Nationalists are not likely to press for an early general election. They will not be eager to join Conservative Party leader Margaret Thatcher, who would very much have to force Labor to the polls now.

The Glasgow election result is no index of a Labor Party victory in a general election, however, and Callaghan would probably be driven from office if a vote was held today. Nevertheless, the outcome in Scotland should strengthen his advisers' view that 1979 is a better year to face the voters than 1978.

Callaghan, and his Labor Party predecessor Harold Wilson, presided over three grim years here, from the end of 1974 to the end of last year. Prices and unemployment rose sharply. Real incomes fell a staggering 6 percent. Callaghan, therefore, has every interest in putting off an election as long as he can, to wipe out this memory and hope that the economy will pick up.

There are signs that times are getting better here. New figures show another slowdown in inflation with the underlying rate now at 6.6 percent or about the European level. Industrial output is slowly recovering and has gained 2 percent in the three months through February.

On Tuesday, Denis Healey, chancellor of the exchequer, announced he was cutting taxes nearly $4.7 billion and increasing spending by about $3 billion on top of that. He has also said that still unpublished figures would show that incomes rose nearly 5 percent at the end of last year, almost to the level from which the Labor government began in 1974.