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Biphasic Sleep: First and Second Sleep, and Does It Work?

What biphasic sleep is, the debate over "first sleep" and "second sleep" in history, what lab studies show about splitting sleep, naps as a midday block, why polyphasic schedules are a bad idea, and who shouldn't try it. For adults.

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Biphasic Sleep: First and Second Sleep, and Does It Work?

Biphasic sleep means sleeping in two blocks instead of one. That can be a night split by a spell of waking, the historical "first sleep" and "second sleep", or a main night's sleep plus a daytime nap. The history is real but disputed, and no trial we found shows that adults who can sleep through the night do better by splitting it on purpose.

This guide is for adults. It covers what biphasic sleep means, what historians argue about first and second sleep, what the science shows, the two modern versions, and who shouldn't try it. This is general information, not medical advice.

What biphasic sleep means

People use "biphasic sleep" in two ways.

A split night. You sleep for a few hours, wake for a while, then sleep again until morning. This is the pattern historians call segmented sleep, or first and second sleep.

A night plus a nap. You sleep a slightly shorter night and add a daytime nap or siesta. This is what most modern "biphasic sleep schedule" articles mean.

Monophasic sleep is the usual modern pattern of one block at night. Polyphasic sleep spreads several short sleeps across the 24-hour day. A National Sleep Foundation panel defined it as multiple short sleep episodes, as opposed to one main sleep and perhaps a nap. Biphasic sits between the two.

First sleep and second sleep: the history

The idea that people once slept in two parts comes mostly from the historian A. Roger Ekirch. In a 2001 article, "Sleep We Have Lost", and later work, he argued that segmented sleep, in two parts with a spell of waking between them, was the predominant pattern in pre-industrial Western Europe. He has since counted more than 2,000 references to it, which he describes as being of varying quality.

Ekirch's picture of a typical night goes like this. Most people went to bed between 9 and 10 p.m. and woke from their "first sleep" around midnight or later. They stayed awake for up to an hour or so on average, then took a "second sleep" until morning. He stresses that the interval wasn't fixed and varied from person to person.

Some of the texts he cites are very practical. One, by the early English writer Andrew Boorde, tells readers to pass water when they wake from their first sleep if their bladder feels full. Ekirch also points to at least 18 medical texts from 17th and 18th-century England and France that named the interval after first sleep as a good time to take pills or potions.

Not every historian reads the sources the same way. In 2023, Niall Boyce re-examined the evidence in the journal Medical History. He argued that "first sleep" had several meanings at the time. It could mean the deep early part of the night, a turn of sleep during a night watch, or the first stretch of a night of broken rest. In one search of early English printed books, he found 15 texts that used "second sleep", and only five of them mentioned both a first and a second sleep. His conclusion was that the references don't, on their own, show that segmented sleep was a routine, widespread habit.

Ekirch replied in 2024, defending his reading. Boyce answered that the question is "by no means settled."

What's fair to say is this. The phrase "first sleep" turns up in many early English printed texts. Ekirch reads the references as showing that waking in the night was familiar. Boyce reads some of them as describing broken, unwelcome sleep. How common a true two-part night was, and how long the waking lasted, is still argued over. "Our ancestors all slept in two blocks" goes further than the evidence.

What the science says

The long-night experiment. In the early 1990s, the psychiatrist Thomas Wehr at the US National Institute of Mental Health moved volunteers from a normal 16-hour day to a 10-hour day, so they spent 14 hours a night in the dark. Their sleep spread out and usually divided into two bouts several hours long, with one to three hours awake between them. As Boyce reports, the fuller 1993 paper had 16 volunteers aged 20 to 36, all but one of them men, with four weeks of long nights.

The conditions were unusual. The volunteers spent the dark hours alone in a windowless room, asked to stay in bed and sleep when they could, with no activities allowed. Boyce points out how different that is from an evening by candle or firelight, and that the split pattern in the paper's figure isn't clearly measured. It shows that sleep can split in very long nights. It doesn't show that a split night is better.

Split schedules in the lab. Researchers studying shift work have tested split sleep directly in adults. In a 2014 lab study of 53 healthy men, average age 26, everyone had 10 hours in bed a day for a simulated five-day work week. Those given one 10-hour night block slept about 8.4 hours. Those given two 5-hour slots slept about 7.2 hours. Performance didn't differ significantly between the groups, but the split group got less sleep, and the night group felt least sleepy. A 2016 study of 16 healthy adults on a 6-hours-on, 6-hours-off pattern found a similar theme: no build-up of performance loss over the four shift days, but less sleep than at baseline and lower alertness on shift days.

In both studies, splitting didn't visibly hurt performance over a few days, but it gave people less sleep. Both were built around shift work, so they say little about choosing to split an ordinary night.

Is the one-block rule settled? Not completely. A 2025 review in Sleep Medicine argued that the scientific foundation for the advice to sleep in one uninterrupted night "may be less canonical than previously assumed", and called for more research on biphasic schedules. That's a call for studies, not a finding that splitting is better.

We searched PubMed on 13 November 2026 for trials of biphasic, segmented or split sleep. In adults, we found shift-work and lab studies like the ones above, and no trial showing that people who can already sleep through the night do better by splitting it on purpose.

Modern biphasic schedules

A night plus a nap

This is the version with the clearest evidence behind it in what we read, because the afternoon dip is real. A 2005 review describes a drop in performance in the mid-afternoon for some people and some tasks, linked to a rise in sleepiness at that time of day. It happens even if you skip lunch, though a high-carbohydrate lunch makes it worse.

A short nap can help with that dip. In a 2006 study of 24 young adults who'd slept about 5 hours, 10 minutes of sleep lifted alertness and performance as soon as people woke, and some of the benefits lasted up to 155 minutes. After a 20-minute nap the benefits took 35 minutes to show, and a 30-minute nap brought a spell of grogginess first. That grogginess, called sleep inertia, usually does not last longer than 30 minutes, though it can last longer if you're sleep deprived. Our guide on how to nap without feeling groggy covers nap length and timing in more detail.

If you'd rather rest without sleeping, NSDR (non-sleep deep rest) is guided rest where you lie still, stay awake and follow a voice. NSDR keeps you awake, so it is unlikely to leave you groggy the way waking from deep sleep can. In a 2026 trial of 60 active young adults, a 25-minute nap raised readiness to perform where 10 minutes of NSDR had no significant effect. Our comparison of NSDR vs napping sets out when each makes sense. Our nap timer can time a midday block.

In the lab studies above, split schedules produced less total sleep than one night block. So think of a midday block as extra rest on top of a full night, not a swap for part of it.

A split night

This means sleeping, waking on purpose or by habit for an hour or so, then sleeping again. We found no trial showing that a deliberate split night helps adults who can sleep through the night. In the two shift-work studies above, split schedules brought less total sleep.

There's a more common situation that looks like this: waking in the small hours and not being able to drop off again. That's usually a problem to fix rather than a schedule to adopt. Our guide to why you wake up at 3am covers the usual causes.

Polyphasic sleep: a warning

Polyphasic schedules split sleep into several short sleeps through the day and night. In 2021, a National Sleep Foundation consensus panel reviewed 22 relevant papers and found no evidence of benefit. It recommended against schedules that significantly cut total sleep or break it into several episodes across the day.

A 2026 lab study of 40 healthy adults aged 18 to 35 compared a schedule of six 20-minute sleeps spread around the clock with one 2-hour sleep, so both groups had the same total sleep opportunity. The spread-out schedule produced less actual sleep and more vigilance impairment, particularly in the morning. The authors note that long-term effects remain to be characterised. We don't recommend trying it.

Who should not try it, and what to do instead

Adding a short nap to a full night is one thing. Deliberately changing your night is another. Don't try to split your sleep if any of these apply to you:

  • You already struggle to get enough sleep.
  • You often wake in the night and can't get back to sleep.
  • You feel sleepy most days even after enough hours in bed.
  • You fall asleep during the day without meaning to.

Each of those is a reason to talk to a GP rather than a reason to change your schedule. Our guide on how to fall back asleep has steps for the night itself, and our guide on how to sleep better covers the basics.

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For ongoing sleep problems, CBT-I (cognitive behavioural therapy for insomnia) is the recommended first treatment. Ask your GP.

If you're struggling, talk to your GP. UK: Samaritans 116 123. US: 988.

This is general information, not medical advice. NSDR is not a treatment for any condition.

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FAQ

What is biphasic sleep?

Sleeping in two blocks in 24 hours. Either a night split by a spell of waking, or a main night's sleep plus a daytime nap.

Is biphasic sleep healthy?

No trial we found shows that splitting your sleep on purpose is better than one good night for adults who can sleep through. In the two shift-work lab studies we cite, splitting didn't visibly hurt performance over a few days, but it meant less sleep overall.

Is it normal to wake up in the middle of the night?

Historians disagree about how routine it was in early modern Europe, so it's hard to call it a lost normal. Lying awake for a long time most nights is worth looking at. Our guide to why you wake up at 3am covers the causes and what helps.

What is first sleep and second sleep?

Phrases from early modern European writing. The historian A. Roger Ekirch reads them as two parts of a night with a waking spell between. Other historians, including Niall Boyce, argue the phrases had several meanings and that a routine two-part night is less certain. The debate is still open.

How long should a midday nap be?

There's no single rule. In one lab study, a 10-minute nap helped as soon as people woke, and some of the benefits lasted up to 155 minutes, while a 30-minute nap brought grogginess first. Our napping guide goes through the options.

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