Why do we refer to morning, afternoon and evening as 'in the morning', 'in the afternoon', 'in the evening' but not 'in the night' instead we say 'at night.'
The bottom line is "it's idiomatic" as mentioned but I can offer the below rationale:
1. The origin of "at night" to indicate a point of time and the usage of prepositions "in" and"at"
In olden times, when the time expression "at night" was originated, night might have been thought as a point of time in the day because there wasn't any activity going on and people were sleeping that time unlike daytime. It represents the dark hours and the late time of the day. But morning, afternoon and evening represent a period of time during the daytime where activities were going on.
In fact, night is a period like morning. This is the main reason of the question because the preposition in is used for time periods. Then, the question can be easily changed to "why not at morning but in the morning" because it seems like times within the day generally take at (at noon, at 5:00, at dawn, at dinner, at night), except the ones that take in with the definite article the (in the morning, in the evening, in the afternoon).
Speaking exceptions, one grammar book says the below for at under the title "exceptions: at, on and by":
At can be used for periods identified vaguely, as in at that time, at breakfast time, at night; also for short holiday periods (at Christmas, at Easter). In BrE, at the weekend is used, but in AmE on the weekend.
A Communicative Grammar of English By Geoffrey Leech, Jan Svartvik
In the end, at and in share a long history for the usages that we talk about and it is mentioned as below in OED (including the earliest example and some relevant examples):
IV. Of time, order, occasion, cause, object.
29. Introducing the time at which an event happens:
a. with the time named.
- c1230 (▸?a1200) Ancrene Riwle (Corpus Cambr.) (1962) 28 Ed al le þe oþre tiden.
- 1586 T. Bright Treat. Melancholie xviii. 114 From 3. at after noone till nine at night.
- a1616 Shakespeare Cymbeline (1623) i. iii. 32 At the sixt houre of Morne, at Noone, at Midnight.
III. Of time.
a. Within the limits of a period or space of time.
With in the day, in the night: cf. by day, by night at by prep. 19b.
- a900 Anglo-Saxon Chron. ann. 709 In foreweardum Danieles dagum.
- 13.. K. Alis. 85 By cler candel, in the nyght.
- a1616 Shakespeare Two Gentlemen of Verona (1623) iii. i. 178 Except I be by Siluia in the night..Vnlesse I looke on Siluia in the day.
2. Chaucer's influence on English and vernacular literature
One of the earliest usages of "at night" is from Chaucer's works. However, there is one earlier usage listed in OED. The following are the definition of at night and earliest examples from OED:
at night: at nightfall, in the evening or night; during the hours of darkness. Freq. designating a specified time.
- c1300 St. Theophilus (Laud) 161 in C. Horstmann Early S.-Eng. Legendary (1887) 293 (MED), Þis cas bi-feol in leinte on a satures-day at niȝt.
- ▸c1387–95 Chaucer Canterbury Tales Prol. 23 At nyght was come..Wel nyne and twenty in a compaignye.
It seems like Chaucer might have a big role for the common usage of this idiomatic expression and the usage of this expression has been continued by other influential people like Shakespeare (example from OED):
a1616 Shakespeare Macbeth (1623) iii. i. 43 Let euery man be master of his time Till seuen at Night.
Additionally, the expression in the night has a long history as well and OED lists as below: (including the earliest example and some relevant examples)
b. in (also †of, †on, †upon) the night : by night, during the night.
- OE Old Eng. Hexateuch: Exod. (Claud.) xii. 12 Ic fare on ðære nihte ofer eall Egypta land.
- a1400 (▸a1325) Cursor Mundi (Vesp.) 6196 Drightin self þam ledd þair wai... Wit firen piler on þe night [c1460 Laud vpon the nyȝt].
- c1480 (▸a1400) St. Theodora 288 in W. M. Metcalfe Legends Saints Sc. Dial. (1896) II. 107 To þat thing has he na sycht þat thocht or don is in þe nycht.
- 1600 Shakespeare Midsummer Night's Dream ii. i. 253 There sleepes Tytania, sometime of the night.
- 1990 N.Y. Mag. 30 Apr. 52/1 A dog howling in the night.
On the other hand, there are instances of at morning, at evening and at afternoon but they are uncommon and literary mainly. (See: Ngram result)
OED mentions the below adverbial phrases with modifying preposition (without article) for morning:
at (also †in, †on, before, till, etc.) morning , from morning till evening, from morning to night, etc. Also with adjective, as all (also each, every, next, etc.) morning. Cf. a-morning adv.
There are examples with at morning and at evening and the earliest usage mentioned is in morning. The below are first three earliest usages and some relevant examples from OED:
- a1400 (▸a1325) Cursor Mundi (Vesp.) 7181 On nighter-tale, or in morning.
- c1400 (▸?a1387) Langland Piers Plowman (Huntington HM 137) (1873) C. xiv. 147 (MED), Maules drowen hem to maules on morwenynge by hem-self, And femeles to femeles.
- c1475 (▸a1449) Lydgate Testament (Harl. 218) 286 in Minor Poems (1911) i. 340 Thamerous foules with motytes and carolles, Salue this sesoun euery morwenyng.
- 1679 Processes Kirkcudbright Sheriff Court No. 282 Otherwayes no master that hath ane servant at night is sure to have them at morning.
- 1849 M. Arnold New Sirens i, I, who in your train at morning Stroll'd and sang with joyful mind, Heard, at evening, sounds of warning.
- 1968 D. Moraes My Son's Father i. 3 At morning the sea was a very pale, indolent colour.
Finally, I'm including the below explanation from a linguistics standpoint (from the book Cognitive and Communicative Approaches to Linguistic Analysis edited by Ellen Contini-Morava, Robert S. Kirsner, Betsy Rodriguez-Bachiller). In summary, night is an exception and is shrunk to a point as a contrastive location in time in the phrase at night and the difference between at night and in the night is explained with examples.
At for temporal messages
...at, designating a point in space, is used for point in time as well:
(37) He arrived at three o'clock; at noon; at midnight; at sunset; at dawn.
Just as a spatial point has no length, depth, or height, a temporal point such as three o'clock and noon, has no duration. A problematic use of at is the phrase at night. Unlike noon, night lasts from eight to twelve hours. Section 3 showed that a three-dimensional location is sometimes shrunk to a point when the message is one of contrastive location. At night is the temporal analog of at Plattsburgh and at Stoneybrook, contrastive location in time:
(38) What do the pretty SMU girls like on their plates?
"Pretty much hamburger, hotdogs, steak and, at night, maybe pizza" Brown, 70361
The contrast in (38) is the different food preferences of the girls at the noon and evening meals. The implication of contrast in at night is better perceived when compared to in the night. Suppose the song 'Strangers in the Night' were titles 'Strangers at night'. The title would no longer suggest two lonely people searching each other out, but a dysfunctional couple: "Companions by Day, Strangers at Night".
In example (39), the two phrases appear in a single passage. The neighbor of a woman who may or may not have committed suicide describes the circumstances of her death:
(39) I had made a habit of calling her at night, from my cottage, just to check. The last night I had called, but the line as always busy and it reassured me...
She was found the day after at the bottom of the cliff. I tried to believe that what must have happened was that, restless, disturbed by this telephone call or whatever, she walked out in the night, as she had a habit of doing...With all that warm rain an the fog it might have been as simple as a loosened rock, a misstep. Brown,1171886
The phrase at night implies 'evenings rather than daytime' for the regular calls, a message of temporal contrast. In the night carries no hint of contrast; it describes the dark three-dimensional space into which the woman walked.
There is an element of idiomatic nuance at play here. "I always brush my teeth at night," describes a mundane event that takes place every evening. "Someone stole my new orchid in the night," has an appropriate and slightly sinister connotation and suggest the event took place during a specific night, the night. Contrast the previous theft with the following, "Police report that car thefts are on the rise, particularly at night." It describes an on-going nocturnal event.
Both at night and in the night are acceptable, though they have slightly different moods. There is no conclusive etymological evidence that I could find, though etymonline suggests:
In choosing between at church, in church, etc. at is properly distinguished from in or on by involving some practical connection; a worshipper is at church; a tourist is in the church.
Following this, it would be logical to claim that something at night is more connected to the time of day than something in the night. Curiously, our usage of in the night tends to be more connected to the noun; e.g. "thief in the night", "things that go bump in the night". These examples make little sense when apart from the night, however, they are quite poetic.
Google's account of their usage shows that the first written instances (that they have record of after 1500) of at night are some 50 years before the first in the night. The in the night was heavily favoured by poets, initially Shakespeare, and could have been constructed for its rhythm.
In conclusion, I could find no evidence explaining why "in the morning/afternoon/evening" evolved, but "at night" remained in its original form. This is obviously not a definitive answer, but I post it so that others may pick up where I left off.
Effective communication sometimes relies heavily on idiom. On that note, I agree with Michael Owen Sartin's answer.
The day is often divided into morning, afternoon and evening, and it is idiomatic to corresponding adverbial phrases of time as in the morning/afternoon/evening, either when referring to habitual occurrences or those that are imminent. For example:
- I usually have coffee in the morning.
- We meet in the evening.
- She's coming in the afternoon.
- "When are you going running tomorrow?" "In the morning."
In regular conversation, other modifiers or indicators (pronouns, etc) are used with morning/afternoon/evening. Sometimes, native speakers drop the articles and prepositions where nonessential to the understanding of the intended recipient. A few quick examples include: tomorrow afternoon, this evening, yesterday morning, morning, etc.
In the night is more formal and poetic, and thus rarely used in regular conversation, at least in my experience. However, it could be used in the same sense as the phrases in the morning, etc. On the other hand, every night is a phrase that is heard very often.
The phrase at night tends to be more common in reported speech and other expressions of habit (... sleep at night is a notable example). Equivalent expressions for other times of day are at dawn/daybreak, at midday/noon, at dusk/twilight. Of these, at noon probably occurs most frequently in spoken conversation.
As @tchrist comments, there are in fact plenty of written instances of at [the] morn/morning, but it's very much a declining/poetic usage.
Contrastingly, at [the] afternoon has never had any significant currency. Because afternoon already contains a preposition of time/location (after), adding another one (at) doesn't work.
And as @Joe Blow comments, in the morning/afternoon/night are common usages (where in is a container metaphor which invariably equates to a duration reference in temporal contexts). As opposed to the locational metaphor preposition at, which in temporal contexts normally identifies a specific point in time.
Thus "We will attack at night" tends to imply at the point where day turns into night, whereas "We will attack in the night" implies any time during the night. But where the reference is to a "continuous, extended" activity (I work at/in the night), both forms are used.
TL;DR: The issue isn't really "Why do we use at for night, but in the for morning, evening?". It's "Why don't we use at for morning, evening?". To which I suggest a major part of the answer is...
Morning and afternoon are much more associated with (and influential towards) each other than they are to night. And at [the] afternoon is awkward (because of the double preposition), which has led to at [the] morning falling out of favour "by association".
It's because we are awake during the day, and asleep throughout most of the night.
Saying in the morning/afternoon means at any time within the period of morning or afternoon.
The same with in the night, if someone said that you would think of any time between the hours of 8pm and 6am, or thereabouts.
However, at night generally means the specific time between when night begins and when you go to sleep, let's say between 8pm and 10pm.
It's similar with other specific times of day, such as at midnight or at noon.
We also do it with the morning, but usually either say at dawn or at the time I woke up instead.
I'm going out at night for a walk.
Generally people would say tonight, but for the basis of this question, saying this sentence would make one think the person is going out late (between 8pm and 10pm).
I'm going out in the night for a walk.
This makes one think they are going out anytime between 8pm and 6am.
"night" can be used with several prepositions:
- at night, in/during the night, by night.
It is up to you which variant you use, the meaning is the same.
With periods of time you normally use "in" or "during". Sometimes dimensions are reduced. When you use "at" you either reduce a time stretch to a point of time or you have the concept of "at some point of the night". "by" is used in connection with light: by candlelight, by moonlight, by the first light of the day" and in analogy "by" can be used with day/night: Paris by night.
in that night
in a specified night
e.g. I saw a ghost while I was on the way going home in that night.
in the night
in a specific time of the night only
e.g. I do my homework in the night before I go to sleep.
throughout the whole night
e.g. The night markets open at night only.
e.g. I sleep at night.
in the morning
in a specific time of the morning only
e.g. I brush my teeth in the morning before I go to school/work.
on the morning
in an unspecified specific morning
e.g. She called me on the morning of 1st June.