People who go to bed feeling lonely and sad wake up with a surge of energy-boosting hormones, a US study has suggested.
Levels of hormones tend to fall at bedtime
A team from Northwestern University, Illinois, studied stress hormone levels in 156 people aged between 54 and 71.
The researchers found bedtime moods affected levels of the hormone cortisol, which can "rev up" the body.
The study appears in the Proceedings of the National Academy of Sciences.
The researchers focused on whether cortisol levels were predicted by people's experiences the day before or from the same day.
Saliva samples were taken three times a day and each participant was asked to write a diary recording their feelings each night.
Cortisol helps the body to raise blood sugar levels and blood pressure in response to stress but also causes changes in mood and memory.
Levels of the hormone are usually high when people wake up and increase over the next 30 minutes. They tend to fall at bedtime.
Emma Adam, who led the research, said: "You've gone to bed with loneliness, sadness, feelings of being overwhelmed, then along comes a boost of hormones in the morning to give you the energy you need to meet the demands of the day."
The study also found people who got angry during the day had higher bedtime levels of cortisol.
"High levels of cortisol in the evening are a kind of biological signature of a bad day," said Ms Adam.
They study also showed people with lower levels of cortisol in the morning felt greater fatigue during the day.
"Stress systems are designed to translate social experience into biological action," Ms Adam said.
"They are designed to be a conduit from the outside world to our internal worlds so that we can better respond to our social context."
Dr Allan Norris, of the British Psychological Society, said: "It certainly sounds plausible - both that cortisol may be a consequence of feeling low and may influence these feelings."
But he said sleeping could also be an important factor in giving someone more energy the following day.
"If someone has gone to bed feeling lonely and overwhelmed, it usually means there is a reason for it," he said.
"People dream about what troubles them and we won't always remember.
"It's as if our mind is resolving the trouble for us."