Ruth L F Leong, Liang Tian, Nicole Yu, Teck Boon Teo, Ju Lynn Ong, Michael W L Chee, Bidirectional associations between the duration and timing of nocturnal sleep and daytime naps in adolescents differ from weekdays to weekends, Sleep, 2024 47(9), zsae147, https://doi.org/10.1093/sleep/zsae147

*Naps may be detrimental to the same night's sleep only if they are long and occur late, as they can delay bedtimes and shorten nocturnal sleep that same night. 

​

Stijn AA Massar, XY Chua, Ruth LF Leong, et al. Sleep, Well-Being, and Cognition in Medical Interns on a Float or Overnight Call Schedule. JAMA Network Open. 2024;7(10):e2438350. doi:10.1001/jamanetworkopen.2024.3835
* In our NUHS Sleep study, we found that a float schedule (consecutive 12 h shifts) may have fewer negative
outcomes for sleep, well-being, and cognition than a call schedule (24 - 30 h overnight shift), and that naps during night work are beneficial.

Ruth L F Leong, TeYang Lau, Andrew R Dicom, Teck Boon Teo, Ju Lynn Ong, Michael W L Chee, Influence of mid-afternoon nap duration and sleep parameters on memory encoding, mood, processing speed, and vigilance, Sleep, 2023. 46(4) zsad025, https://doi.org/10.1093/sleep/zsad025
*Compared to wake, all nap durations ranging from 10 to 60 min had clear benefits for positive mood, subjective sleepiness, and alertness that lasted up to 4 h post-nap. While even a short 10-minutes can act as a midday refresh, a 30-min nap appears to have the best trade-off between practicality and benefit. 

Alyssa S.C. Ng, Stijn A.A. Massar, Bei Bei, Michael W.L. Chee, Assessing ‘readiness’ by tracking fluctuations in daily sleep duration and their effects on daily mood, motivation, and sleepiness, Sleep Medicine, 2023 Volume 112,Pages 30-38, https://doi.org/10.1016/j.sleep.2023.09.028
*Positive connections between nocturnal sleep and nap duration with mood, motivation and sleepiness are temporally circumscribed, lending credence to the construction of sleep-based, daily ‘readiness’ scores.

Ruth L.F. Leong, June C. Lo, Michael W.L. Chee, Systematic review and meta-analyses on the effects of afternoon napping on cognition, Sleep Medicine Reviews, 2022. Volume 65, 101666, https://doi.org/10.1016/j.smrv.2022.101666
*Reviewing 60 experimental studies across different cognitive tasks, we found that afternoon naps have a significant small to medium benefit over multiple cognitive tests. Benefits were strongest for vigilance and memory. Nap benefits transcend age and nap duration.

Hosein Aghayan Golkashani, Ruth L F Leong, Shohreh Ghorbani, Ju Lynn Ong, Guillén Fernández, Michael W L Chee, A sleep schedule incorporating naps benefits the transformation of hierarchical knowledge, Sleep, 2022. 45(4). zsac025, https://doi.org/10.1093/sleep/zsac025
*A sleep schedule incorporating regular nap opportunities, compared to one that only had nocturnal sleep, benefited the building of robust and flexible schemas. This facilitated the recall of subsequently rearranged and expanded knowledge structures.

James N. Cousins, Ruth L. F. Leong, Azrin S. Jamaluddin, Alyssa S. C. Ng, Ju Lynn Ong, Michael W. L. Chee. Splitting sleep between the night and a daytime nap reduces homeostatic sleep pressure and enhances long-term memory. Sci Rep 2021. 11, 5275 https://doi.org/10.1038/s41598-021-84625-8
*In our Need for Sleep 4th iteration, we found that naps could be incorporated into a daily sleep schedule that provides sufficient sleep and benefits learning.

Ruth L F Leong, Nicole Yu, Ju Lynn Ong, Alyssa S C Ng, S Azrin Jamaluddin, James N Cousins, Nicholas I Y N Chee, Michael W L Chee, Memory performance following napping in habitual and non-habitual nappers, Sleep, 2021. 44(6).  zsaa277, https://doi.org/10.1093/sleep/zsaa277
*Contrary to concerns that napping may be disadvantageous for non-habitual nappers, we found that an afternoon nap was beneficial for long-term memory tasks even if one did not habitually nap. Naps were especially beneficial for habitual nappers, as it restored the memory decline that would otherwise have been incurred without a midday nap.

June C Lo, Ruth L F Leong, Alyssa S C Ng, S Azrin Jamaluddin, Ju Lynn Ong, Shohreh Ghorbani, TeYang Lau, Nicholas I Y N Chee, Joshua J Gooley, Michael W L Chee, Cognitive effects of split and continuous sleep schedules in adolescents differ according to total sleep opportunity, Sleep, 2020. 43(12)., zsaa129, https://doi.org/10.1093/sleep/zsaa129
*In the 5th and last iteration of Need for Sleep, we showed that the effects of split sleep on basic cognitive functions vary by the amount of total sleep obtained. As long as total sleep opportunity over 24 h is within the recommended range (8-9 h), students may fulfil sleep requirements by adopting a split sleep schedule consisting of a shorter period of nocturnal sleep combined with a mid-afternoon nap, without significant impact on basic cognitive functions.

Ju Lynn Ong, Te Yang Lau, Xuan Kai Lee, Elaine van Rijn, Michael W L Chee, A daytime nap restores hippocampal function and improves declarative learning, Sleep, 2020  43,(9),  zsaa058, https://doi.org/10.1093/sleep/zsaa058
*Participants who napped encoded word pairs better than a comparable group who stayed awake in the same period. Increases in hippocampal activation following the nap suggest restored hippocampal function, and naptime spindles may contribute to improved memory.

Cousins, J.N., van Rijn, E., Ong, J.L. et al. Does splitting sleep improve long-term memory in chronically sleep deprived adolescents? 2019.  npj Sci. Learn. 4, 8. https://doi.org/10.1038/s41539-019-0047-z
*Under conditions of sleep restriction, a split sleep schedule benefits learning after a nap opportunity without impairing morning learning, despite less preceding nocturnal sleep. While not replacing adequate nocturnal sleep, a split sleep schedule may be beneficial for chronically sleep restricted learners.

June C Lo, Derek C K Twan, Swathy Karamchedu, Xuan Kai Lee, Ju Lynn Ong, Elaine Van Rijn, Joshua J Gooley, Michael W L Chee, Differential effects of split and continuous sleep on neurobehavioral function and glucose tolerance in sleep-restricted adolescents, Sleep, 2019. 42(5), zsz037, https://doi.org/10.1093/sleep/zsz037
*In the 4th iteration of Need for Sleep, we showed that multiple nights of sleep restriction had divergent negative effects on cognitive performance and glucose levels depending on how the restricted sleep was apportioned (continuous nocturnal, or nocturnal + nap). Teens are best advised to obtain the recommended amount of nocturnal sleep.

Ong JL, Lo JC, Patanaik A, Chee MWL. Trait-like characteristics of sleep EEG power spectra in adolescents across sleep opportunity manipulations. J Sleep Res. 2019 Oct;28(5):e12824. doi: 10.1111/jsr.12824. 
*The trait-like characteristics of electroencephalographic spectra from an adolescent remain identifiable despite the disruptive effects of multi-night sleep restriction schedules to sleep architecture.

Cousins JN, Van Rijn E, Ong JL, Chee MWL. A split sleep schedule rescues short-term topographical memory after multiple nights of sleep restriction. Sleep. 2019 Apr 1;42(4):zsz018. doi: 10.1093/sleep/zsz018.
*Using our Need for Sleep data, we found that hippocampal function is sensitive to moderate multi-night sleep restriction, but deficits can be ameliorated by splitting sleep between nocturnal and daytime, at least for a period after waking from the daytime nap. 

Cousins JN, Wong KF, Raghunath BL, Look C, Chee MWL. The long-term memory benefits of a daytime nap compared with cramming. Sleep. 2019 Jan 1;42(1):zsy207. doi: 10.1093/sleep/zsy207. 
*Young adults learned detailed factual knowledge in sessions that flanked 1 hr spent napping, taking a break , or cramming that information. When tested 30 min after learning, cramming and napping led to significantly better memory than taking a break. After a week, napping maintained this significant advantage, but cramming did not.

Lo JC, Lee SM, Teo LM, Lim J, Gooley JJ, Chee MW. Neurobehavioral Impact of Successive Cycles of Sleep Restriction With and Without Naps in Adolescents. Sleep. 2017 Feb 1;40(2):zsw042. doi: 10.1093/sleep/zsw042.
*In our second iteration of Need for Sleep, we found that adolescents' neurobehavioral functions may not adapt well to successive cycles of chronic sleep curtailment and recovery. In sleep-restricted adolescents, weekend "catch-up sleep," even when combined with napping during weekdays, is inferior to receiving a 9-hr sleep opportunity each night.

Lim J, Lo JC, Chee MW. Assessing the benefits of napping and short rest breaks on processing speed in sleep-restricted adolescents. J Sleep Res. 2017 Apr;26(2):219-226. doi: 10.1111/jsr.12497.
*In Need for Sleep 2, we studied how napping and rest breaks (interleaved off-task periods) might ameliorate the negative effects of sleep restriction on processing speed. The findings suggest that napping partially reverses the detrimental effects of sleep restriction on processing speed.